



























































CHARLES A. SPENCER, 


Greatest American Optician,—Born, 1813; Died, 1881 


[ See page 225 






































■4 




II 






























































HAND-BOOK 

FOR 

OPTICIANS. 


A TREATISE ON THE OPTICAL TRADE, AND ITS 
MECHANICAL MANIPULATIONS. 


BY 


7 


W. BOHNE, 

n 

OPTICIAN. 


{Second JSdition 


THOROUGHLY REVISED AND GREATLY ENLARGED. 


WITH ILLUSTRATIONS. 


q,OPVR 

UG 1 •; 


r/ 


PUBLISHED BY THE AUTHOK, 

(With A. B. GRISWOLD & CO.; 

No. 119 CANAL STREET, NEW ORLEANS, La. 
1892. 







35i 



» 


Entered according to Act of Congress, in the year 1892, by 
THE AUTHOR, 

In the Office of the Librarian of Congress, at Washington, D. C. 


* 


> "FBESS” PBINT, 50 BIENVILLE ST., N. O. 

* 


J 







PREFACE TO SECOND EDITION. 


Since the 4 4 Hand-Book for Opticians ” made its first 
appearance, I have, to my greatest satisfaction, observed 
a general wholesome stir among the opticians, manifest¬ 
ing itself by several new publications in the same line; 
by the increased attendance of young opticians at the 
different Ophthalmic Colleges, and by the issue of a 
44 Monthly Journal ” in our interest. This was my 
reason for taking another step in the further instruction 
of my companions. The first edition was merely a feeler 
to ascertain if a book, so different from other instruction- 
books, was wanted or rather needed. The favorable re¬ 
ception it received, even outside of the trade, induced 
me to extend its usefulness by adding some information 
which I purposely omitted before, judging that the 
medical faculty would properly attend to the theoretical 
part of our occupation. But their writings demand a 
partially scientific education which most of us, simple 
opticians, have not received. My explanations may not 
be strictly professional; but a diligent reader will readily 
understand them, and — that is, in my opinion, the prin¬ 
cipal object of all instructions. 

The treatise on the Development of the Optical Trade 
(Chap. XXVI), although enlarged, is still insufficient in 
its present state ; and to compensate for its short-com¬ 
ings, I have added the next chapter, in which the gradual 
progress of the optical science is individualized by a brief 
history of the lives of those men, who attributed to the 
advancement of our trade and science in general. 

The large space I devoted to the memory of the late 
Charles A. Spencer may be a surprise to many opticians 



4 


PREFACE. 


who perhaps never heard of him; he is better known in 
medical circles than among his own trade-companions; 
better in Europe than in America. Even the cyclopedists 
have neglected him ; he went to the grave almost un¬ 
known to his neighbors. Spencer was the first optician 
who produced objectives of wide angles, and inspired the 
studies of scientific men in all parts of the world. With¬ 
out his genius, many of the marvelous discoveries accom¬ 
plished by the microscope could never have been made. 
Our country did herself a great wrong in not making 
more of her gifted son, and it is the sacred duty of the 
American opticians to prevent his name from being for¬ 
gotten. 

It is with great pleasure that I acknowledge my indebt¬ 
edness to Dr. H. D. Bruns, Mr. H. Ginder, Mr. Chas. F. 
Prentice, Mr. J. M. Johnston and Mr. G. C. Kidgway, 
for their valuable assistance and kind advice in the prep¬ 
aration of this work. 

New Orleans, 1892. 

W. BOHNE. 


EXTRACT FROM PREFACE OF FIRST EDITION. 


My object is to instruct the rising generation of our 
trade, and elevate them to the position of the great pro¬ 
gress optical science has made within the last quarter of the 
century. I am well aware that the present work is not 
as complete as it ought to be, because every chapter is 
composed and written as something new. There is noth¬ 
ing previously published about these subjects, and my 
book may be the pioneer to open the road for other writ¬ 
ers. Almost every trade has its literature or hand-book 
of the secrets peculiar to its business; but the optical 
trade, as regards the mechanical part of it, has none 
whatever. 

What I offer here is the result of a life-long experience 
and of numerous investigations. Workmen who find any 
error, or who know better methods, are cordially invited 
to communicate their information to the author, who will 
acknowledge his obligation in a future edition. 

Let us remove the curse of all progress—the keeping 
of our secrets and little tricks to ourselves. Let every 
workman withdraw the restriction placed upon his fel¬ 
low-laborers, forbidding them to enter his shop, in order 
to prevent them from profiting by his skill. This is the 
proper way to elevate our trade to a commanding posi¬ 
tion, so that we may no longer be confounded with street- 
fakirs and mere spectacle-vendors. 

My book will furnish to any young man a solid foun¬ 
dation of what he ought to know, and will enable him to 
master all difficulties he may encounter in the pursuit of 
his occupation. As there is no telling what demand will 
be laid on his ability in the immediate future, he should 



0 


PREFACE. 


try to understand thoroughly the fundamental laws of his 
trade and become a competent workman. 

Chapter Y explains all about the optical line and center 
in lenses, and chapter VII, of the setting of compound 
lenses; both proved for many years to be the stumbling- 
block of our efficiency and ability. Chapter III treats of 
pebbles , but differs from anything heretofore published. 
I hope that my experiment will be repeated by opticians 
and scientists in order to finally settle the vexatious ques¬ 
tion: “Shall pebbles be used or not?” I am anxious to 
hear what others have to say about them. 

The history of the “Invention and Introduction of 
Spectacles,” is the first attempt at collecting the scanty 
materials about this important subject, and is far from 
being what its title indicates. Those of my readers, who 
are in possession of facts concerning this matter, will 
kindly communicate them to me for future use. 

New Orleans, 1888 . 

W. BOHNE. 


CONTENTS. 


PAGE. 

CHAPTER I.—Inch and Metric Systems. 9 

H.—Different Qualities of Lenses. 19 

III—Merits and Defects of Pebbles. 31 

IV.—Prisms, Spherical and Cylindrical Lenses 40 

Y.—Optical Line and Center ..54 

VI.- —Setting of Spherical Lenses. 58 

VII.—Measuring and Setting of Compound 

Lenses. 65 

“ VIH.—Selection of Spectacles. 71 

“ IX.—Double Focus Single and Split Glasses. 76 

“ X.—Colored or Tinted Glasses. 82 

“ XI.—Redressing of Spectacle Frames. 88 

“ XII.—Use of Test-Types. 90 

“ X7TL—Pefraction and Dispersion of Light ... 96 

“ XIV.—Achromatic Lenses. 102 

“ XV.—Anatomy of the Human Eye .. 108 

“ XVI.—Presbyopia, Hypermetropia and Myopia 116 

“ XVII.—Astigmatism. 134 

“ XVITL—Ophthalmoscope. 141 

“ XIX.—Second Sight. 147 

“ XX.—Relief to Injured Eyes.. ... 152 

“ XXI.—Artificial Human Eye. 156 

“ XXH.—Caloric Rays in Different Lights. 162 

“ XXIII.—Range of Vision. 171 

“ XXIV.—Tears . 175 

“ XXV.—Facial Expression. 179 

“ XXVI.—History of the Invention of Spectacles, 
and Gradual Development of the 

Optical Trade. 184 

“ XXVII.—Prominent Opticians, Scientists and In¬ 
ventors . 201 

“ XXVIII.-Miscellanies.. 232 

“ XXIX.—Glossary. 238 

Index. 247 



























I +0 




ABBREVIATIONS. 


D = diopter, 

ax = axis. 

C. or cyl. = cylindrical, 
cc = concave, 

cm = centimeter, 

m = meter, 

mm = millimeter. 

S or sph. = spherical. 

= concave. 

= convex. 

— combined with. 

= degree. 

= foot, also a minute. 

— inch, also a second. 

= line, the twelfth part of an inch 



CHAPTER I. 


Inch and Metric Systems. 


Spectacle lenses are made of glass or pebbles ground 
to a spherical form, either convex or concave, by means ' 
of tools which are segments of a ball or sphere. 



The dotted lines represent the whole spheres of which 
but segments in form of shells or cups are employed for 
the grinding of lenses. If we use the inside or hollow 
part of the shell, we produce a convex lens, while the 
outside or rounded part is employed for concave lenses. 



Some lenses are curved only on one side, while the 












10 


HAND-BOOK FOR OPTICIANS. 


other side is flat; they are called plano-convex or plano¬ 
concave. 



Periscopic lenses are ground by spheres of different 
sizes, as shown in the above cuts. 

We have, therefore, three kinds of cx, and three 
kinds of cc lenses. Cx lenses collect behind them, 
by refraction, the greatest portion of the rays falling on 
their surface at one common point, called the “positive 
focus,” which is nearer to, or further from the lens 
according to its focal power. A concave lens, on the 
contrary, disperses or scatters the rays, and has a “neg¬ 
ative focus,” because we only can neutralize it by a 
plus or positive focus lens. The term “negative lens” 
does not exactly cover the nature of a concave lens; 
we can measure its focus also by reflection, which gives 
in front of it a positive focus, just as a convex lens 
will do behind it when measured by refraction. For 
instance, fasten a white card at the end of a foot-rule, 
go to a glass-door or window, hold the ruler so that the 
card is between the window and the lens, approach the 
concave lens until you get a sharp image of the window 
on the card; then see on the ruler how many inches the 
lens is from the card, and if it is a double concave, 
multiply by two, if plano-concave, by four, and you 
have the focal length of the lens in inches. Periscopic 
lenses cannot be measured this way. Although the latter 
are highly recommended by their inventor, Wollaston , 
and by other celebrated authorities, I, for my part, find 
that the stronger numbers are extremely unpleasant to 
the eye, especially when they are used for cataracts. 
All that is claimed for their superiority may be granted 
to the weaker numbers from one to four diopters , but 
not for stronger ones. 










INCH AND METRIC SYSTEMS. 


11 


Here arises the question, which of the three words is 
right and should be used: Dioptric , Dioptry or Diop¬ 
ter ? These three scientific terms are derived from the 
Greek verb dioptomai ( dia , through, and optomai , I 
see), I see through, I see completely. In Modern 
Greek dioptres means spectacles. 4 4 Dioptrique’ ’ (English 
“dioptric” ) has been adopted by the French in connection 
with optical measurements, as a substitute for the term 
meter , which latter, although denoting measure in gener¬ 
al, has no specific application to optical measurements. 
Originally, the French word dioptrique, both by deriva¬ 
tion and common use, does not include any idea of 
measure or measurement whatsoever, it only refers to 
the refraction of light. It is simply an old word with a 
new technical meaning, contrary to the logical rules of 
language, like many other words of foreign extraction. 
Dioptrique not being a noun but an adjective, and not 
sanctioned by scientific usage, at least not yet in the 
English speaking world, we should exclude it henceforth 
from our optical terminology in regard to measurement. 
Likewise objectionable is the noun 4 ‘Dioptry” as a term 
for expressing optical measures. 

The word diopter , considered as a contraction from 
“dioptometer,” is evidently the most suitable for our 
purpose, and is strictly analogous to words like barometer, 
thermometer, etc., also employed for different measure¬ 
ments. The word “diopter” also denotes a geometrical 
instrument used for leveling purposes—The substitution 
of the word diopter for meter has been adopted by oculists 
and ail first-class opticians since 1875, in order to in¬ 
troduce a uniform measurement instead of the old inch 
measure, which considerably differs in length in the 


different countries. So is 

1 Paris inch.27.07 mm. 

1 English inch. 25.3 44 

1 Austrian inch..26.34 44 

1 Prussian or Rheinish inch. .. 26.15 44 

and one meter contains 37 Paris inches, 

“ 44 44 39.37 English inches, 

44 “ 44 38 Austrian inches, 

“ 44 44 38.23 Prussian inches. 


a 





12 


HAND-BOOK FOE OPTICIANS. 


This explains why imported lenses never correspond 
with our numbers and have to be remeasured. When 
we order No. 20 ( 2 V)* we find them generally to be 22; 
and it is only by keeping in stock the half numbers, as 
54, 64, etc., and odd numbers like 17, 19, etc., that we 
are able to fill the orders of oculists. The trouble is 
increased that some oculists have their test-lenses meas¬ 
ured by the French inches, some by the Eheinish meas¬ 
ure, others by the English or American, and as the same 
differences occur in the measurement of lenses offered by 
different manufacturers, the confusion is a general one. 

This trouble is definitely overcome by the introduction 
of the metric system , as the meter is independent of the 
special measurements of different countries. The inch 
system has also the great inconvenience that the unit 
represents a lens of 1 inch focus, and that we have to 
express the strength of ail lenses weaker than No. 1 in 
fractions. A lens which we call No. 10, is really one 
tenth as strong as No. 1, and has to be written yq, and 
two lenses of this strength combined are X V = } or 
No. 5. But this was not the only disadvantage of the 
inch system; we were also obliged to carry an unneces¬ 
sary assortment of numbers in stock which were utterly 
useless. I only mention here the numbers of lenses found 
in the catalogues of importers from 40 going up in even 
numbers to 60. Such numbers as 42, 44, 46, etc.,, are 
of no earthly use, as will be seen by the following table, 
showing the differences in strength between those num¬ 
bers which the better informed opticians kept in stock. 
The differences between 


5 

and 

54 in 

inches 

is in 

diopters 0.73 

54 

6 i 

6 

i ( 

1 

6 6 > 

C 6 

0.60 

6 

l i 

64 

i i 

1 

7 8 > 

i t 

0.51 

64 

i i 

7 

i ( 

1 

9 1 5 

i i 

0.44 

7 

6 i 

74 

i i 

1 

10 5) 

i i 

0.38 

74 

6 t 

8 

i i 

1 

12 0 5 

i i 

0.34 

8 

i i 

9 

i t 

1 

7 2 5 

6 i 

0.56 

9 

i i 

10 

t i 

1 

9 0 5 

( ( 

0.44 

10 

i 6 

11 

t i 

TTo'5 

(( 

0.36 

11 

i i 

12 

i i 

T3~2 5 

( ( 

0.30 

12 

i S 

13 

a 

1 

15 6 5 

( ( 

0.25 


INCH AND METRIC SYSTEMS. 


13 


13 and 

14 is 

in inches 

ln 

diopters 

0.22 

14 


15 

< t 

1 

2105 

i i 

0.19 

15 

t < 

16 

6 i 

1 

2 40 ) 

t i 

0.17 

16 

i i 

18 

i t 

1 

1 4 4 5 

a 

0.28 

18 

i t 

20 

i i 

1 

1805 

i i 

0.22 

20 

i i 

24 

i t 

1 

12 0 ? 

i i 

0.33 

24 

i i 

30 

i i 

1 

120 ) 

i i 

0.33 

30 

i t 

36 

t ( 

1 

1805 

i i 

0.22 

36 

i i 

40 

i ( 

3 " 6"0 » 

i i 

0.11 

40 

t 6 

48 

i ( 

1 

2 40 5 

i i 

0.17 

48 

i ( 

60 

i t 

1 

2405 

i i 

0.17 

60 

( ( 

72 

< t 

1 

3 605 

( i 

0.11 

72 

( ( 

90 

i i 

1 

3 6 0 ) 

i i 

0.11 


In order to have a full understanding of the above 
table, let us take a patient who complains of his spec¬ 
tacles being too weak. We find them by measuring to 
be No. 10. If we combine with them our weakest inch- 
number in stock, No. 90, we increase their focal strength 
to No. 9. — Another patient is wearing No; +12; he 
finds them too .strong, but is well pleased after we have 
added — 0.25 diopter. According to the above table 
we find that we have decreased the strength of his spec¬ 
tacles from + 12 to + 13. — The differences between 
two numbers, from No. 13 up, are not much more than 
i diopter (0.25 D), in some cases less than J diopter, 
and the differences of all the intermediate numbers, 
not mentioned in this list, are so little that they amount 
to almost nothing, and should be omitted in our stock of 
lenses as unnecessary. 

Spectacle lenses are manufactured in such quantity 
and so cheap, that we cannot expect them to be mathe¬ 
matically correct like the lenses of scientific instruments, 
else they would be considerably dearer. The careless 
way in which many people put on their glasses is no 
encouragement to manufacture perfect and high-priced 
lenses; they would only prove to be a waste of labor 
and money. Most people have no more use for such 
perfect spectacles than an Indian has for classic music. 
But there is one essential requirement without which a 
lens is totally worthless, i. e. they should be well 
centered. The center of a lens is the highest or lowest 


14 


HAND-BOOK FOR OPTICIANS. 


point of its surface (according as it is convex or con¬ 
cave); and as each side is finished separately, these 
centers should be exactly opposite each other, so that a 
line drawn through them is at a right angle with the 
plane of the lens from edge to edge through its middle. 
The test for a w T ell centered lens is to get in sunlight a 
sharply defined small circle, and the smaller this circle 
the more perfectly the lens is centered. All common 
spectacles are badly finished in this respect, and should 
not be sold by any conscientious optician. If people 
are willing to injure their eyesight for the sake of a few 
dimes, let them do so; but we should rather lose the 
sale of a good article than be parties to such reprehensible 
dealings. The traffic in “eye-killers” should be pro¬ 
hibited by law, as druggists are forbidden to sell poison 
without discrimination. 

The first qualification of an optician is his ability to 
measure lenses in inches as well as in diopters. The 
inch system is not so simple as many, opticians consider 
it to be; it includes some scientific points which are 
based on the refractive power of the different sorts of 
glass (Chap. XIII). The same curve, or radius of 
curvature, does not always produce the same focal power. 
If the grinder takes, for instance, slabs of flint, crown 
glass and pebble, and finishes them with the very same 
mould or segment of a ball, he will find that his lenses 
are of different strength, because the index of refraction 
of those three kinds of material is of different power. 
But this scientific distinction between the “radius of 
curvature” and the “actual focal power” of a lens is of 
little'importance to the average optician, who has only 
to do with its focal distance, indicated either in inches 
or diopters. Let us, therefore, turn our attention to the 
practical method of measuring lenses by the focal dis¬ 
tance in inches, and ignore entirely by what curve the 
lens was ground. There is a simple way to determine 
if a lens is convex or concave, which, in weak numbers, 
is sometimes a difficult task for an inexpert eye. For 
this purpose we hold the lens a few inches from the eye, 
and look through it at a distant object; then we move 
the lens slowly to the right and left, or up and down, 


INCH AND METKIC SYSTEMS. 


15 


and when the object apparently moves in the same direc¬ 
tion as the lens, it is concave; if it moves in the opposite 
direction, it is convex. Even the weakest lens, say 0.25 
D., shows a distinct movement one way or the other. 
These movements are more easily detected in weak 
lenses when they are held at a greater distance from the 
eye. 

If you have in your store or workshop a suitable place 
to fasten permanently a rule of 40" in length, horizon¬ 
tally, with a white card attached at zero, and counting 
from that point in the direction of a conspicuous object, 
for instance a window or a railing, 20 or more feet away, it 
is easy to find the focus by moving the convex lens back 
and forth upon the rule until you' have a well-defined 
picture of the window on the card. This figure is always 
reversed, the reason for which will be explained in the 
next chapter. As soon as the figure shows clearest, you 
observe on the rule the number of inches, which will be 
also the number of the lens. There is no difficulty in 
measuring in this way convex lenses up to 30"; beyond 
that, it requires greater care and some practice to dis¬ 
tinguish the faint picture on the card, especially in cloudy 
weather. The surest way to measure weaker lenses than 
30", is to place two together and measure them conjoint¬ 
ly. Two lenses of 48 will give 24, and one lens separate 
will be again 4 V or half the strength of But if we 
have two lenses apparently of the same strength, which 
are actually 60 and 72, how can we ascertain that they 
are of different strength ? The ruler will be useless to 
us, even if we would lengthen it sufficiently. Here we 
have to fall back on our own eyes, and let them render 
judgment. Take for instance a folding foot-rule of 2', 
and open it sufficiently to introduce a pin-head between 
the open ends. You will hardly think that this little 
opening has much effect on the parallelism of the two 
lines. But when we place this foot-rule so that the 
continuation of one branch strikes a point several 
hundred feet away from us; then, without moving the 
rule, and following with our eye the other line, we will 
see what effect this little opening has. You will under¬ 
stand by this, that you have to compare such weak lenses 


16 


HAND-BOOK FOR OPTICIANS. 


by looking at remote objects. If there is convenient a 
roof of a house one or more hundred feet away from 
you, take the two lenses, 60 and 72, one in each hand, 
hold them edgewise together, and look through them at 
arm’s length at the roof; move one or the other lens up 
or down till you see the lower line of the roof straight 
through both glasses. Now look, without moving the 
lenses, at the upper line, and you will find that it is 
higher in one lens, which is, of course, the stronger one 
(No. 60), as it is of greater magnifying power. 

As regards the measurement of concave lenses, we 
have to deal principally with their “negative” nature, 
the opposite of the “positive” character of convex 
lenses. The comparatively easy task of providing our¬ 
selves with a full set of convex test-lenses, facilitates 
the otherwise difficult job to determine their strength 
by means of reflection of the weaker numbers. Besides 
we would be entirely helpless in this regard without the 
assistance of convex lenses, in measuring periscopic 
concave lenses, which cannot be done at all by reflection. 
But with a full set of convex lenses we can readily 
neutralize concave lenses of any strength and shape, and 
find their exact focal power without the least trouble. 
If you have a concave lens of which you do not know 
the number, pass before it different convex lenses till 
you come to that one which makes them both together 
appear plane, and the number of the convex lens is the 
number of the concave one: -f- -f and — y is 0, or plane. 

The principle on which the manufacture of periscopic 
lenses is based, needs a short explanation. The peculiar 
shape of a periscopic lens, also called. Meniscus, is in¬ 
dicated by “concavo-convex,” and its strength is the 
sum of its relative curves. If one side represents — 2 V 
(— 2D), and the other + i ( A 5 D), we have a lens 
of periscopic convex + ^ (+ 3D). If we reverse the 
signs, so that one side is + yo and the other — J, then 
we have a periscopic lens of — -jV, or — 3D. A little 
reflection will convince us of the absurdity of making 
cataract lenses in this form. The extreme convexity of 
one side will cause such an aberration of light, that only 
the smallest center-part of the lens will be useful and 
pleasant to the eye. 


INCH AND METRIC SYSTEMS. 


17 


The unit of the metric system is based on the length 
of a meter, which is 39.37 American inches, and is ex¬ 
pressed by the sign 1 D. Therefore, two diopters will 
be = 19.68", and 3 D == 13.12". But to simplify their 
calculations and to avoid the complicated fractions, we 
may take the meter at 40 inches, and we will be near 
enough for all practical purposes. The annexed table 
will show the nearest approximation of both systems; 
the small numbers give the exact value of diopters in 
American inches: 

Diopters: 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 

Inches: 160 80 53 40 32 26 28 20 18 16 

157.48 78.74 52.49 39.37 31.49 26.24 22.49 19.68 17.49 15.74 

Diopters: 2.75 3.00 3.25 3.50 4.00 4.50 5.00 5.50 6.00 6.50 

Inches: 14J 13 12 11 10 9 8 7 6| 6 

14.31 13.12 12.11 11.24 9.84 8.75 7.87 7.15 6.56 6.05 

Diopters: 7 8 9 10 11 12 13 14 16 18 20 40 

Inches: 5| 5 4| 4 3£ 3£ 3 2J 2£ 2£ 2 1 

5.62 4.92 4.37 3.93 3.57 3.28 3.02 2.81 2.46 2.18 1.96 0.98 

Of these numbers there are only 53, 23 and 14J not 

common to the old inch system, but the next numbers 
in inches may be substituted till we can provide for their 
equivalents in diopters. I do not recommend to fill the 
orders of oculists in an inaccurate way, but we have seen 
before, that the difference between two numbers or half 
numbers amounts to very little, and is of no consequence 
to the wearer. 

The great advantage of this new measurement is that 
it enables us to make calculations and combinations of 
lenses without the least trouble, while the old way in 
inches is more or less difficult. Suppose somebody is 
wearing + A> but cannot see well, and we add another 
lens + 4 V, by which combination he sees perfectly; then 
we have to calculate in this way: 

A + A — T§f<r + AA — rofo — A (about), or 16 
inches. In diopters it is quite simple: A = 1 D, and 
3 ^ == 1.50 D, and both together 2.50 D =* 16". 

To give another illustration of the difficulty in making 
correct combinations by the inch system, we will take an 
achromatic objective lens, composed of a crown glass of 
4 £ inches, and a flint glass of 7§ inches. These two 



18 


HAND-BOOK FOR OPTICIANS. 


lenses combined give a lens of + 10" focus, and the 
regular way of making the calculation is this: 

+ 4* V **= 1 ^ + V- ^ '+ A 
- 7| = - 3 “ — 1 — ‘T 7== — Af 

by reducing these fractions to a common denominator, 
which is 299, we get for + A~ == + Anr, and for — A 
— — A 9 > added together gives + At or A inch focus. 
All this trouble is avoided by turning the above lenses 
into diopters: 

+ 44 equals very nearly + 9 D 
— 7f “ “ — 5D 


added together equals + 4 D, or + A”* 
We have seen that the word diopter is simply a sub¬ 
stitute for a meter, which has in America 39.37", and in 
Paris 37". Only the inches are of different length, not 
the meter, and to find, for instance, the difference 
between an American and a French foot, we have to 
multiply the numbers of lines in one foot (144'") by 37, 
and divide the product by 39.37; the quotient will be 
135.3"', which is the length of an American foot in 
French lines. The French foot is, therefore, 8.7'" 
longer than the American foot. To reduce inches to 
diopters, we divide them into 39.37 (or 40), and the 
product will be diopters; if we divide the diopters into 
40, then the product will be inches. 



CHAPTER II. 


Different Qualities of Lenses. 


Many opticians make a mistake about the comparative 
hardness of flint glass and crown glass, and even noted 
writers fall into this error. Dr. Donders, for instance, 
says: “Flint glass and rock crystal are harder than 
crown glass.” I do not understand how this mistake 
could slip into so many medical books, as the simple 
test of scratching the one with the other will show at 
once that crown glass is harder than flint glass. Dr. 
Donders is not so much to be blamed for his incorrect 
statement as those who have reproduced his error with¬ 
out any investigation. I read lately in a valuable 
geographical work of Dr. H. Berghaus, that Geo. 
Washington served his country twelve years as President. 
I do not think less of Dr. B. for making this erroneous 
statement, but I censure every writer who quotes him 
as an authority on the subject. 

We find another error in regard to pebbles, repeated 
in books written by careless compilers without examin¬ 
ing the facts; and as the first writer was mistaken, all 
the rest labor equally under the same gross misrepresen¬ 
tation. I will correct in the next chapter this error 
which has, for years, caused an open contest between 
oculists and opticians. The oculists based their objec¬ 
tions on books of high authority, and the opticians 
yielded to their argument from sheer want of correct 
information. I warmly urge both to devote some of 
their leisure time to investigate this question thoroughly, 
and try to settle it definitely. 




20 


HAND-BOOK FOR OPTICIANS. 


Quartz is the principal ingredient in the manufacture 
of glass, which is the most transparent of all solid sub¬ 
stances produced by man, and also the best imitation of 
that valuable product of nature, termed rock crystal, 
“pebble,” or crystallized quartz. The scientific name 
for it is silex, but when it is combined with an alkali or 
another mineral, it passes under the name silica , forming 
with them the so-called silicates', so glass is a silicate of 
potash and lime. Quartz is composed of fifty per cent, 
of oxygen with about an equal proportion of its base, 
silicium , which is supposed to be a metal like potassium 
and sodium; but chemists cannot yet reduce it to its 
metallic state. In the year 1827, the base of clay was 
extracted in the form of that extremely light metal, 
aluminium (generally written aluminum , although the 
latter is but the Latin name for clay, and not for its 
metallic base); but the metal silicium is still waiting for 
its discoverer.* 

To manufacture glass , we must take quartz or sand, 
— the latter is only powdered or crushed silex — and 
melt it together with either potash or soda with the 
addition of lime, borax, lead and other ingredients 
which facilitate its fusion. Quartz or sand by itself will 
never melt, being perfectly infusible, but it acquires the 
property of fusibility to a greater or lesser degree 
according to the quantity of the above metallic oxides 
with which it is mixed before undergoing the melting 
process. There are many formulae published for the 
manufacture of glass, but as we shall see, not every 
kind is fit for optical purposes. 

The word “glass” derives from the Saxon verb glis-nian 

* A commencement in “this direction has been made in electroplating 
with silicium, obtained directly from quartz by means of hydrofluoric and 
hydrochloric acids. The metal silicium is invisibly suspended in the 
solution in which the article to be plated is immersed, and is set free by 
the action of a galvanic current. In this way we obtain a thin film of the 
real metallic base of quartz. An incandescent lamp has lately appeared 
in England in which the filament is coated with a layer of silicium and 
the degree of vacuum required inside the bulb, it is claimed, can be 
lessened. — When we consider how tedious were the first experiments 
with Aluminium, and in what quantity and with what facility this metal 
is novy produced, we may also expect to see Silicium introduced sooner or 
later into the market as a new metal for ornamental or industrial pur¬ 
poses. r 



DIFFERENT QUALITIES OF LENSES. 


21 


(the German gleissen ), to shine, to be bright, and was 
by old writers frequently applied to shining or glittering 
substances, without reference to color or transparency. 
The combination of sand with an alkali (potash or soda), 
melted together, yields only the so-called water-glass , 
soluble in boiling water to a fine, transparent, semi¬ 
elastic varnish, used for the adulteration of soap; also 
for hardening mortar cements, etc., so as to render them 
impervious to water. An application of it to wood 
renders the same almost incombustible. But to produce 
a glass not to be affected by water and acids, it is 
necessary to add one or more of the metallic salts, such 
as barium, strontium, calcium, magnesium, aluminum, 
manganese, arsenic, or lead. Some of these facilitate 
the melting process, and are called fluxes or solvents, 
others serve as decoloring agents. The proportion in 
which the above substances are used, and the different 
compositions made by them in addition to the principal 
ingredients (quartz and potash) constitute the different 
kinds of glass. Although there is no secrecy of the 
most improved formulae of making all kinds of glass, 
it is nevertheless a well established fact that one factory 
offers a better article for sale than others, because they 
take better materials, and their workmen are more 
careful and competent, as is the case in all other trades. 
It is almost impossible to obtain or even prepare the 
ingredients in a state of chemical purity previous to 
fusing them together. 

Sand is always more or less impure, and must be 
carefully washed and cleaned. Many varieties of this 
material are not fit for the manufacture of glass. Only 
rock crystal and quartz are chemically pure, especially 
the first, but they require the extra expense of being 
pulverized. Formerly flint (silex), calcined and ground, 
was used as the source of the silica for the manufacture 
of fine glass; hence the name of flint glass. 

Potash and Soda* are used in a purified state only 
for the best qualities of glass, but crude potash and 
soda-ash are employed for the medium quality, while 


* They are often combined, as their mixture acts more rapidly, and 
at a considerably lower temperature than either of them will separately. 



22 


HAND-BOOK FOR OPTICIANS. 


common wood-ashes and refuse soda will do for bottle 
glass. The potash used for this purpose is the carbonate 
of potash (Salt of Tartar), and requires a process of 
washing previous to use. The state to which it is 
brought by the process of cleaning is that of fine white 
grains, differing but little, to an unpracticed eye, from 
the prepared sand. Other combinations of potassium, 
such as nitrate of potash (saltpeter), and sulphate of 
potash (alum), counteract the tendency to color, before 
the glass enters into perfect fusion. 

Lime (calcium) forms an important part in the 
manufacture of glass, and may be introduced either 
slaked, burned or as a carbonate (chalk). Limestone, 
however, that contains iron, must be excluded from the 
mixture for making white glass. The action of lime is 
to promote the fusion of the mixture, and to make the 
glass hard. 

Lead is the distinguishing ingredient in crystal, com¬ 
mon flint glass, optical glass and strass. It is used in 
the form of red lead or litharge, and removes many 
impurities as, for instance, charcoal by oxidation. An 
excess of lead induces too great softness in the glass, 
and gives a yellow tinge. 

Baryta (barium), in the form known as “heavy 
spar,” is sometimes added to the constituents of com¬ 
mon bottle glass to render it more easy of fusion. 

Aluminum (clay), though seldom purposely intro¬ 
duced into glass, is always accidentally present, brought 
there by the action of the materials upon the clay of the 
pots in which they are melted. If present in any 
quantity it spoils the perfect crystallization of the glass. 

Iron is another unwelcome element, which is almost 
always present in the sand, in the soda and in the chalk, 
and produces a greenish color in the glass when not 
removed. 

•Arsenic, in little quantities, promotes the decom¬ 
position of the other ingredients, and tends to dissipate 
carbonaceous impurities not otherwise disposed of, but 
is then volatilized. In excess, it produces a milkiness 
in the glass, which time will increase. 

Manganese is employed to neutralize the greenish 


DIFFERENT QUALITIES OF LENSES. 23 

tint produced by the presence of iron, and to counter¬ 
act the impurities of carbon. If particles of carbon or 
soot from the fire or flame become mixed and sur¬ 
rounded with the melted glass, these, by their exclusion 
from the access of air, are prevented burning, and a 
brown or smoky color is produced, which is removed 
by the conversion of the carbon into carbonic oxide 
through the oxidizing influence of manganese. (Arsenic 
and nitrate of potash are also used for the same pur¬ 
pose.) From the cleansing action of this material, it is 
generally termed the “glass-makers’ soap.” It must, 
however, be used sparingly; for an excess of it gives an 
amethystine tint to the glass. Such colored lenses are 
introduced into the market under the name of “Arun¬ 
del.” 

Borax is sometimes employed as a flux, but it must 
be used always with great caution, as an excess leads to 
exfoliation of the glass. 

There are four varieties of glass manufactured, besides 
the above mentioned water glass : 

1. Flint glass , also called Crystal , jS trass or Paste. 
This is a very pure and beautiful kind of glass, of great 
density and high refractive power. It is properly 
termed lead-glass, since it is the presence of this metal 
which distinguishes it from all other varieties of glass. 
It is chiefly manufactured into articles of domestic use 
and ornament, and is an English invention.* The best 
formula of flint glass manufactured for optical pur¬ 
poses is: 


42.5 parts of silica or sand, 

43.5 “ 

“ oxide of lead, 

11.7 “ 

“ carbonate of potash, 

1.8 “ 

“ nitrate of potash, 

5 “ 

“ chalk. 


^_ 100 . 

* Flint glass was known over three hundred years ago. There was 
as early as 1557 a factory of it in London, and English flint glass was 
considered the best in the market. But they never could make pieces of 
more than a few inches in diameter, suitable for astronomical purposes, 
till Fraunhofer astonished the world with a lens of almost a foot in diam¬ 
eter, which was set afterwards into a refractor for the observatory at 
Dorpat, in Russia, and is yet in use. The difficulty is that the great 
quantity of lead in flint glass cannot be equally distributed throughout 
the lens. 




24 


HAND-BOOK FOR OPTICIANS. 


The flint glass well prepared is almost without color. 
It excels in brilliancy and in refracting power all other 
glass, and when well polished by the lapidary, is con¬ 
sidered the nearest approach to the diamond; but it is 
soft and easily scratched. The specific gravity is 3.7, 
due to the great quantity of lead, while that of crown 
glass is 2.7, and of rock crystal only 2.6. Many opti¬ 
cians may have confounded density with hardness, and 
have made the same mistake as if they maintained, that 
dense and compact chalk was harder than light and 
porous pumice stone, although the hardness of the first 
is 1, and of the other 7. Density is the opposite of 
rarity, not of softness.* 

2. English Grown Glass , Plate and Window Glass. 
Crown glass is also an English invention. It was in¬ 
troduced into the market in circular plates, from which 
particular form it received its name. It does not con¬ 
tain any lead, and is therefore much lighter than flint 
glass. It has gained its great reputation since the 
invention of achromatic lenses for telescopes and other 
optical instruments. As the value and beauty of this 
glass depend entirely upon its absolute limpidness, a 
most careful selection of materials, and a protracted 
and assiduous attention of the workmen are required. 
The difficulties in producing large, thick masses of this 
glass for optical purposes are greater to-day than the 
fabrication of similar pieces of perfect flint glass; 
because the melting process of the ingredients of crown 
glass requires a greater temperature than those of flint 
glass, on account of the absence of lead, and, by adding 
instead more alkali to increase the facility of fusion, it 
becomes liable to attract humidity, or as it is technically 
termed, to sweat, which would make it unfit for optical 
instruments. The best formula of this glass for our 
purpose is: 

* The problem of the manufacture of good optical flint glass in large 
pieces was first solved by F. Guinand, a Swiss watchmaker, who joined 
Fraunhofer’s establishment at Munich. Both kept the process a secret. 
The superiority of their glass is considered not to have been in the novelty 
of the materials or their proportions, but in the careful agitation of the 
liquid glass, while at the highest point of fusion; then in the cooling down 
of the entire contents of the pot in a solid mass, and afterward in separ¬ 
ating unstriated portions by cleavage. 



DIFFERENT qualities of lenses. 


25 


White sand.120 parts 

Carbonate of potash. 35 “ 

Carbonate of soda. 20 “ 

Chalk or slaked lime. 20 “ 

Arsenic. 1 “ 


196 parts. 

v There are 55 parts of alkalies in 196, which is equal 
to 28%, against 12% in flint glass. The addition of 
chalk is very essential; it counteracts the effect of the 
excess of alkalies which would produce in the lenses a 
constant tendency to tarnish, by the deposit of a film of 
aqueous vapor, and would cause them in the space of a 
few years to lose their polish. 

The name of Plate glass , or, as it should be termed, 
cast-glass , might be applied to any kind of glass in 
sheets. This beautiful kind of glass is formed by being 
cast upon a smooth marble table while in a liquid state, 
and is totally independent of the process of blowing. 
The principal consumption of plate glass is for mirrors. 

Window Glass is also a crown glass of inferior quality, 
which is first blown at the end of the pipe into a large 
globe, then converted by a rapid rotatory motion into a 
cylinder, which is cut up in the direction parallel to its 
axis and flattened into a broad sheet. The materials 
employed for the manufacture of this glass are chiefly 
silica, soda and lime; no potash is used. 

3. Bohemian or Crystal Glass. The coarser qualities 
of this kind of glass are analogous in composition to 
bottle glass. But the finer kinds are distinguished by 
their comparative freedom from color, by great light¬ 
ness, and their very refractive nature, which renders 
them capable of resisting not only high heats, but sudden 
changes of temperature. Hence the value of this glass 
for chemical purposes, such as retorts, tubes, etc. Its 
lightness and the total absence of color cause it to be 
highly valued as table-ware, for costly windows, cover¬ 
ing of engravings, and also for spectacle lenses. In the 
finer qualities of Bohemian glass, potash is substituted 
for soda. Its formula is: 








26 


HAND-BOOK FOR OPTICIANS. 


Quartz in powder.100 parts. 

Carbonate of potash ..... 60 “ 

Carbonate of lime.. 20 “ 


4. Bottle Glass. The materials for common glass 
bottles are coarser than for any other kind of glass, and 
consist of silica, lime, soda, oxides of iron and man¬ 
ganese. Economy is the chief object, color and appear¬ 
ance being of no moment. The colored sands are even 
preferable to white sands, because the oxide of iron, 
which colors them, performs the part of a flux. They 
do not require any washing or other preparation, and 
instead of soda, common wood ashes obtained from 
domestic fires will do, after they are sifted and dried 
before Using. This glass is the hardest of all, but is the 
most impure and therefore useless for optical purposes. 

We see from this list, that the dearer potash is used 
only for the better qualities of glass, and the cheaper 
soda for the inferior kinds. If either of them is em¬ 
ployed too freely, it spoils the glass. You have perhaps 
made the observation, after having carefully cleaned a 
mirror or window, that soon there was a scum again 
covering the just brightened surfaces. A repeated rub¬ 
bing readily removed it, only to reappear as soon as you 
ceased your efforts. The excess of potash or soda 
attracts the moisture of the air, and baffles your exer¬ 
tions. Don’t laugh any more at people complaining 
that they can never clean their spectacles; the lenses 
may be manufactured of such defective glass. 

There is another serious evil attending the excess of 
alkalies in glass; they gradually oxidize by the action 
of the atmosphere, causing the appearance of rain-bow 
colors. But when after a length of time the potash and 
soda are more and more absorbed from the surface, 
there is left only a thin film of oxidized silica of a milky 
appearance, such as you find on spoiled lenses, especially 
on watch glasses, which are mostly made of glass con¬ 
taining too much soda. Such lenses cannot be cleaned by 
any acids, or by any amount of rubbing with water. The 
only way to clean them is a gentle rubbing with Cosmo- 
line, provided they are not too far gone. 

I think it proper here to direct your attention to the 





DIFFERENT QUALITIES OF LENSES. 27 

many so-called inventions of unscrupulous parties, 
introducing their wonderful discoveries as something of 
great importance, i. e. for their own pockets, not for 
the public. The only invention they have really made 
is the high-sounding name which they flourish osten¬ 
tatiously before the eyes of the amazed public; then 
waiting eagerly for the rush of deluded buyers, as the 
picadores in the arena wait for the headlong advance of 
the bull, enraged by the waving of the red cloth. Such 
names as Perfected, Improved, Brilliant , Arundel , 
Diamond , Medicated, Diamanta , Crystal, Parabolic, 
etc.,* are still fresh in our memories, and the list will 
increase as long as there are enough dupes living to 
make such a humbug pay. — I hope nobody will mis¬ 
construe these remarks as if I were against lawful 
advertisements of a good article. There is a way of 
introducing goods which is perfectly honorable, provided 
such parties use their own name as a recommendation 
for the superiority of their spectacles. If Dick & Harry 
are competent opticians, and keep nothing but first-class 
goods, nobody can blame them for drawing public 
attention to the fine brands of “Dick & Harry’s Optical 
Goods.” Their success is due to their expert selections 
and superior judgment, and not to false pretentions. 

It is, therefore, very important that every optician be 
well informed about the different qualities of lenses ; he 
should be able to determine their various grades as read¬ 
ily as a jeweler is able to ascertain the karats of goods 
he is buying. Lenses of th q first quality always contain 
more or less lead, the larger its quantity (to almost half 
its volume), the finer its lustre and beautiful sparkling. 
This kind is known to the trade as extra white Hint glass, 
and cannot be distinguished from pebbles by simply 
comparing them together by look. It is principally 
used for opera glasses and other optical instruments. 

The best method of comparing different lenses is to 
place them horizontally or flat between your fingers; by 


* The latest fake is a Southern product, called “Crystallized Lenses.” 
I think, the South has as much right to humbug people as the North and 
East of the U. S. Who will be the pioneer in the West? There is a 
general demand for such inventions. 


3 



28 


HAND-BOOK FOR OPTICIANS. 


holding the hand towards the light, you can see in the 
narrow open spaces between your dark fingers the dif¬ 
ferent degrees of the color of these lenses better than 
by placing them on white paper. But most lenses sold 
for first quality are not the extra white, and cannot stand 
comparison with pebbles; the simple hand-test shows a 
grayish tinge when compared with them. 

Lenses of the second quality are either of crown glass 
or the refuse of flint glass, and are of course less costly. 
If they are made of a clear, Well finished crown glass, 
they are preferable to any flint glass, because they are 
harder, take a higher polish, and are for this reason 
more suitable for cataract lenses. The inferior kinds 
of crown glass are also used for spectacles of lower 
grades, but the lenses have a greenish tinge when 
examined edgewise, and are full of imperfections plainly 
seen when looked through at the sky. Such lenses seem 
to be filled with half transparent little particles of dust, 
due to the incomplete process of melting the sand. 

The third quality is not always made, of poorer glass, 
because many lenses from the better qualities are selected 
to be used as they were cast. We find, therefore, among 
them very often white lenses, but they are never ground, 
and seldom polished. Their cheapness is due more to 
saved labor than to less costly material.—I could extend 
the list of the different qualities to fourth and fifth 
grades, when I look around among the stock in trade of 
peddlers and “street opticians,” but I hope none of my 
readers will be caught selling such trash. It is true, the 
eye can stand a great deal of abuse, but the wearer of 
such spectacles will at last share the fate of a spendthrift: 
the one loses his fortune, the other, alas, his sight. 

To detect other imperfections we have to hold the 
lens at an angle of 35° in good light. The reflected light 
will show the smallest bubble or scratch in or upon the 
glass. Another and better method is to hold the lens 
before the eye, and look through it at a window. (This 
test refers only to cx lenses.) We will see the object 
behind it dimly, and in lengthening the distance grad- 
ually, it will appear still dimmer, till at once we see 
nothing but the glary lens, — it is just in its focal dis- 


DIFFERENT QUALITIES OF LENSES. 


29 


tance. If we remove the lens beyond this point, the 
object is then clearly seen but reversed, because the 
rays have crossed in the focus; the upper rays are now. 
the lower ones, and vice versa. This point where we 
see nothing behind the lens, is the most proper for 
detecting all imperfections in the lens. 

I conclude this chapter with some suggestions of reform 
to my associates in the optical trade. Every merchant tries 
to buy as cheap as possible in order to meet competition 
on an equal footing. Our constant demand for lower 
prices compels the importers and jobbers to make a 
similar request to the manufacturers, who, of course, 
will produce inferior goods to satisfy the general press¬ 
ure. The* consequence is a gradual decline in the 
quality of goods. What we call to-day “first quality,” 
but pay only one quarter of what we paid thirty years 
ago for it, is not the same article.. I still have from 
that time lenses on hand of Nos. 21, 23, 25, 33, 45, 50, 
etc. (because I thought them necessary to be well 
assorted), which are yet as bright as new lenses. Since 
they have fallen in price, I am compelled every year to 
throw many lenses away, even among those just received 
from an importing house, because some show already 
traces of rain-bow colors, others even are corroded. In 
former years, the grinders always had great trouble to 
find the right quality of glass for optical lenses, but 
since they can dispose of all kinds of trash, they work 
up any stuff which never was manufactured for that pur¬ 
pose. Glass, barely good enough for table-ware or 
window glass, is turned into spectacle lenses, because 
they readily can be sold to one or the other party. If 
our importers themselves were scientific opticians, or had 
one in their employ to superintend this branch of 
their business, the general decline in the quality of 
lenses would have been prevented. They would have 
found it to their interest to always keep a stock of good 
lenses on hand, even if they had to provide themselves 
for the trade at large, for jewelers and peddlers, with 
imitations. At present it is impossible to find the 
genuine lenses made of real optical glass. I do not 
blame the importers alone, but confess that this state of 


30 


HAND-BOOK FOR OPTICIANS. 


affairs is mostly our own fault. We made the great 
mistake of altogether disregarding our responsible 
position to assist mankind in the preservation of their 
precious eyesight with faultless glasses; we degraded 
ourselves to mercenary traders for the sake of gain. 

I remarked before, that we should be able to classify 
the different grades of lenses as readily as the jeweler as¬ 
certains the karats of his gold. I myself experimented 
for years in analyzing the different qualities of spectacle 
lenses as to the quantity of lead, arsenic, clay, potash, 
soda and other materials used in their manufacture, but 
I succeeded only partially by tedious chemical processes, 
which are not yet of any practical value to the craft. I 
wish others would direct their attention to this highly 
important subject, and being more successful, will earn 
a deserved reputation by publishing their discovery. 


CHAPTER III. 


Merits and Defects of Pebbles. 


For more than a hundred years after cotton began to 
be cultivated in America, its seeds were considered worth¬ 
less, and on every plantation large heaps of this con¬ 
temned stuff accumulated in the course of time, which 
the planter would have gladly given for nothing, if any¬ 
body had been kind enough to cart it away. To-day, the 
seed yields more profit to him than the cotton. Pebbles 
met with the same treatment. Neither the builders had 
any use for them, nor street-pavers; only mineralogists 
noticed them, and occasionally collected some specimens 
as cabinet-pieces. A few manufacturers of glass also 
used them for making an extra quality of flint glass, but 
millions of tons of this precious mineral were left un¬ 
noticed by those who are now eagerly searching for it. 
Since 1783, when Alexis Rochon, the first writer on 
pebbles, gave an unfavorable account of them, and con¬ 
demned them as useless for spectacle lenses, all writers 
on the subject are against them. Listen to what a 
Doctor says: 4 

“The only practical advantage of pebbles over glass is, 
that they enable us with all honesty to gratify persons 
who do not know what they want, but simply wish to pay 
more than the usual price, or more than their friends did 
for their spectacles.” 

Another says: 

“Rock crystal, or Brazilian quartz, is also used, and 
is commonly known as pebbles. It has no advantage 
over glass, except in hardness; in fact, the opticians 
find it difficult or impossible to distinguish between them 
without a polariscope or a file. Many people, however, 
are not satisfied unless they have pebbles, or think they 
have them, for glass is very often sold instead.” This 




32 


HAND-BOOK FOR OPTICIANS. 


physician forgets that jewelers are also compelled to use 
touchstone and acid to test gold. Is gold, therefore, less 
valuable ? 

I have frequently tried to find some information about 
pebbles; but being unable to discover any book or 
pamphlet treating this subject, either here or in Europe, 
I concluded to search for myself, and ascertain if there 
was anything in it to repay the labor. A superficial 
glance at them revealed their extreme transparency, 
and plainly showed that few spectacle lenses possessed 
that brilliancy which characterizes these crystals. I 
asked myself the question: Why should we abandon 
the natural, pure glass for an artificial substitute; the 
reality for an imitation ? 

The genuine article has two striking advantages over 
glass which cannot be denied: its brilliancy and its 
hardness. The principal objection made against the use 
of pebbles is their double refraction ; but this is seen 
only in thick pieces, when we look through their slightly 
inclined surfaces. Objects thus seen through polished 
planes of massive pieces appear double, which is not the 
case with thinner plates, like spectacle lenses.* 

Since, therefore, double refraction will affect vision 
only in thick pieces and not in thin ones, what reason 
have opponents to prejudice the public against their use ? 
Why not raise their voices likewise against the use of 
small quantities of arsenic, belladonna and other poi¬ 
sons ? for it is well known that large doses of them have 
deadly effects. On the contrary, they, as well as the 
most cautious and conscientious physicians, daily pres¬ 
cribe small doses of these poisons, with successful results. 

This is the only serious objection ever made against 
pebbles, and I would think it too insignificant in compari¬ 
son with their other high qualities, which give them a 
prominent place among all their competitors for spectacle 

* “The double refraction of rock crystals renders them useless for 
optical purposes, and especially for the manufacture of spectacle lenses, 
and although the images do not appear double across such lenses in con¬ 
sequence of their thinness, and the manner in which they are used, it is 
nevertheless true that double refraction exists, and that it can cause consid¬ 
erable trouble to vision by weakening the retina, and producing fatigue of 
the accommodation, or even a kind of amblyopy.’’—Manuel de l’Etudiant 
Oculiste, par Arthur Chevalier. Paris, 1868. 



MERITS AND DEFECTS OF PEBBLES. 


33 


lenses, to waste another word in their defence, if it were 
not my object here to settle the dispute definitely, and 
furnish all the points necessary to justify my honest, 
favorable opinion about them. The main object of my 
investigation was to ascertain if the eyes were fatigued 
sooner with pebbles than with glasses. I directed my 
attention especially to the general cause of our getting 
weary, and I found that it is the effect of heat which 
relaxes the muscles and produces the sensation of fatigue. 
Consequently, those lenses which will transmit the most 
light and, at the same time, the least heat to the eye, 
should be used for spectacles. The test which I made 
in this respect by means of thermometers, first in 1871, 
I repeated before writing this article. In order to make 
this test simultaneously with different lenses, I selected 
six thermometers which worked accurately together; then 
I took an axis pebble , a non-axis pebble , a flint glass , a 
crown glass , a light smoked and an Arundel lens , all of 
+ 8. I made a slender frame-work to hold the lenses 
and the thermometers; then removing the thermometers 
from their casings, I placed them, one each, in the foci 
of the different lenses. To guard against any inequality 
in this test, I took a straight piece of sheet-iron, and had 
six holes punched out, all of the size of a silver quarter 
dollar, and fastened the lenses behind each hole so that 
the optical center of the lens was in the center of the 
hole. 

I took altogether thirty-two observations, with the 
following result: 

The smoked lens showed 
“ crown glass 


non-axis pebble 
axis pebble 
flint glass 
Arundel lens 


78° 

81° 

814 

82° 

83° 

84° 


on the average. 


The lesson we may draw from these observations is 
that we should dispense with flint glass and all colored 
lenses, except smoked. Crown glass and pebbles are 
then left as the only rivals for spectacle lenses. 

To thoroughly ventilate the question: ‘‘Shall pebbles 
be used or not,” it is necessary first to have a full un- 


34 


HAND-BOOK FOR OPTICIANS. 


derstanding of the properties of crystals in general, and 
then to consider the difference between axis and non-axis 
pebbles. — Crystals are divided into: Single refracting 
crystals, such as rocksalt, alum etc., and double refract¬ 
ing crystals, of which we have two kinds: 

1. those with a single optic axis, as Iceland spar, rock 
crystal, tourmaline, beryl, etc., and 

2 . those which possess two optic axes, as feldspar, 
mica, topaz, etc. 

When we take a plate or slab of a double refracting 
crystal with the single optic axis, for instance, Iceland 
spar, we see the object through its axis single; but when 
we hold it obliquely, the object appears double. We see 
then two distinct pictures, one produced by the ordinary 
ray which shows the object single when we look through 
its axis, another object by the extraordinary ray, separ¬ 
ating itself from the first one, the more so when gradual¬ 
ly we incline the surface of the crystal towards its 
equator, or perpendicularly to its axis. This is called 
the double refraction of a crystal. The two objects 
separate from or approach to each other, according to 
the position of our eye towards the equator, or towards 
the axis of the crystal. But when we turn the inclined 
crystal around its equator, from right to left, or from 
left to right, keeping the position of its poles unchanged, 
then the extraordinary image of the object rotates around 
the ordinary one without altering the relative distance to 
each other. As pebbles belong to this class of crystals 
we make a memorandum of their first property: A pebble 
has no double refraction in its axis. 

Let us now determine what is an axis , and what is a 
non-axis pebble. Bock crystals are six sided prisms 
terminated by six sided pyramids. 

The dotted line from apex to base of the pyramid, 
parallel to the sides of the prism, indicates th q principal 
axis of the crystal. It is evident that this axis is not 
always in the middle, but very often to one side of the 
crystal according to the position of the apex. Lenses 
cut at right angle to the axis from crystals where the 
principal axis is in the center of the column , will show in 
the polarizer (tourmaline plates) perfectly shaped colored 


MERITS AND DEFECTS OF PEBBLES. 


35 




rings whose center is in the middle of the lens. But 
when the apex is at one side, all lenses cut in the same 
manner of such crystals will show the prismatic colors 
only at one side of the lens and generally very faint and 
imperfect. Lenses cut at right angles to the principal 
axis are called axis pebbles. In case the lenses are cut in 
any other direction, such lenses are called non-axis pebbles, 
and do not show between the tourmaline plates the rain¬ 
bow colors. If any one of my readers will examine his 
stock of axis pebbles, he will be astonished how few 
perfect lenses there are among them, perhaps not one in 
a hundred. All talk about the preference of axis pebbles 
is, therefore, imposition, because they are not in the 
market. But suppose they were manufactured according 
to scientific principles, and could be bought even at a 
great increase of price on account of the few perfect 
crystals found, would it be worth while to bother our¬ 
selves about them ? Let us see what double refraction 
in pebbles amounts to. The greatest power in this res¬ 
pect is in Iceland spar, and plates of it for experimental 
purposes are generally one inch thick, in order to show 
their action to some advantage. The thinner these slabs, 
the Jess will be their action. A plate of one line (or the 
twelfth part of an inch) will have very little effect. To 
produce the same effect as such a thin plate of Iceland 
spar will have, we must take one of rock crystal 115 
times as heavy, which would be a lens of 10 inches thick. 

Is it not ridiculous to warn people of double refraction 
in pebbles ? We could with equal right warn them not 
to breathe, because there is carbonic acid in the air. 
(One molecule of air contains .79 nitrogen, .21 oxygen 
and .0004 carbonic acid). 








36 


HAND-BOOK FOR OPTICIANS. 


And so we make another memorandum to the credit of 
rock crystals: Double refraction in pebbles is perceptible 
only in very heavy plates. 

It now remains to compare pebbles with crown glass 
and see in whose favor the scales will turn. Crown glass, 
as we have seen in the preceding chapter, is a hard glass, 
sufficiently clear for optical purposes, but it shows edge¬ 
wise a greenish tint. Some manufacturers produce a 
crown glass without this tint by adding during the melt¬ 
ing process, arsenic and manganese in greater proportions 
than usual, which do not interfere with the hardness of 
the glass, as lead would do, but favor its early corrosion. 
If it is made according to the best formula, the index of 
refraction is 1.538, and the index of dispersion 0.037, 
while Hint glass has an index of refraction of 1.633 and 
of dispersion 0.049. When we take into consideration 
that a greater index of refraction dazzles the eye more 
than a. lower one, and a greater index of dispersion an¬ 
noys and fatigues the eye, we understand at once why 
crown glass is superior to hint glass for spectacle lenses. 

As regards pebbles, we have to notice their slightly 
greater index of refraction (1.548), which accounts for 
the higher stand of the thermometer in the trial-test. The 
difference in the refraction of crown glass and pebbles is 
very small and is fully balanced by the lower index of 
dispersion, which is only 0.026 in pebbles. The differ¬ 
ence of the thermometer between axis and non-axis 
pebbles puzzled me at hrst considerably; but I think it 
can be fully explained by the presence of the prismatic 
colors in axis pebbles. The red ray is very predominant 
in such lenses, and as the red is the caloric ray “par 
excellence,” it explains the greater heat in comparison 
to non-axis pebbles. I believe this also covers the case 
in regard to Arundel lenses, which are based altogether 
upon a wrong theory. When we resolve the light by a 
prism into its seven colors and examine the caloric of the 
violet ray, we find it of much lower temperature than that 
of the red ray.* 


* Hersekel found the following degrees of heat in the different colors, 
of the spectrum : 


Violet . 540 

Blue.56 0 

Green.... 58 o 


Yellow.62 0 

Red . 720 

Beyond Red. 790 










MERITS AND DEFECTS OF PEBBLES. 


37 


Bat when we produce a violet lens and let the light pass 
through it, the red ray receives an additional force from 
the reddish tint of the lens, which sends a warmer light 
to the eye than white glass does. In the spectrum, the 
violet ray is isolated from the other rays and is, therefore, 
cooler; but a violet lens does not exclude the other six 
colors. Hence, we cannot expect the same loss of tem¬ 
perature, which depends upon the isolation of the ray 
and not upon the color of the lens. 

The prejudices against the use of pebbles for spectacle 
lenses are of such long standing and are so deeply rooted 
in the minds of many oculists and opticians, that I do not 
expect to have removed all objections raised against 
their usefulness. I am far from representing my opinion 
to the craft as infallible; I content myself with the con¬ 
viction of having done my duty in disenchanting the 
“Sleeping Beauty” and in defending the rights of a 
neglected “Cinderella.” My arguments are like a wet 
sponge, clearing this natural, genuine glass from the dust 
of a century, and enabling it to prosecute its own case 
with the prospect of gaining it before an unbiased court 
of investigation. Its hardness and clearness surpasses 
any glass ever offered to take its place; its dispersing 
power is the lowest of all lenses manufactured for optical 
purposes, and its double refraction is practically harm¬ 
less : who dares to throw a stone at my humble supplicant 
for recognition ? 

Pebbles are particularly fitted to correct cases of pres¬ 
byopia and hypermetropia, because the eyes subjected to 
these deficiencies are rather benefited by the slightly 
increased index of refraction, while the less refracting 
crown glass is preferable for myopia and cataract. Pebbles 
are too°glary for a near-sighted eye and may show traces 
of double refraction in thick, heavy cataract lenses. Eyes, 
sensitive to light, should also abstain from using pebbles; 
only light smoked lenses and, in special cases, light blue 
ones can be used with satisfaction. The light blue tint 
has no disagreeable effect, and is almost indifferent to 

the feeble eye. , ,, 

In closing this chapter, I express the hope that other 
writers may investigate the subject and at last free this 


38 


HAND-BOOIv FOR OPTICIANS. 


4 ‘out-cast” from the odium of being a disguised enemy 
to the eye. As far as I have investigated the matter, I 
find that all the ignorance of thoughtless writers hereto¬ 
fore has not been able to rob these crystals of their hard¬ 
ness, nor to obscure their brilliancy and clearness. All 
insinuations about their double refraction have been 
unable to double the finest test-line or dot in spectacle 
lenses of this mineral, or to produce the great trouble in 
the eyes of the wearer which they so earnestly predicted. 
Pebbles are used to-day and will be used in the future 
as long as rock crystals can be found. I advise all 
opticians to sell them without the least hesitation, but to 
dispose only of those lenses which are faultless as to their 
crystallization. There are many pebbles in the market 
full of imperfections, so-called “watermarks”; they 
should not be used, but thrown aside. 

Lenses of rock crystals were first introduced in England 
under the name of Scotch pebbles, and afterwards as 
Brazilian pebbles, which designation only indicates the 
country from where they are imported and not a difference 
in the quality. Crystals are found all over the world, but 
not everywhere in large pieces suitable for our purpose. 
Bochon discovered in Madagascar a fine quality of crys¬ 
tals, and ever since large quantities of them are brought 
to France, where the optician Canchoix at Paris, 1831, 
manufactured telescopes with lenses of a combination of 
rock crystals and glass, which came into favorable notice. 
But the difficulty of obtaining crystals in proper shape 
and size has been a great obstacle to their general manu¬ 
facture. 

Many peasants of the Valley of Chamounix, Switzer¬ 
land, make it their chief occupation to hunt for rock 
crystals. In the hope of gaining sudden wealth by finding 
a cave full of beautiful rock crystals they peril life and 
limb in scaling dangerous precipices, or hanging suspended 
over frightful abysses, searching wherever they may 
catch a glimpse of the silver-white vein in the granite rock, 
the sign that near by is a deposit of this precious 
mineial. A Swiss peasant, some years ago, realized his 
hope; he found a granite cave from which he took over 
one hundred crystals, the first weighing about 120 pounds. 


MERITS AND DEFECTS OF PEBBLES. 


39 


This was brought to America, and is at present in Phila¬ 
delphia; another one of 265 pounds was kept at Berne, 
the Capital of Switzerland, but is not as fine a crystal as 
the one first mentioned. The largest groupe of rock 
crystals, weighing nearly 1000 pounds, is in the museum 
of the University at Naples. At Milan is a single crystal 
of 3^ ft. long and 5J ft. in circumference, estimated to 
weigh 870 pounds. 

In the United States some rich deposits have been met 
with at Lake George and at Trenton Falls, N. Y., in 
Moose Mountain, N. H., in Waterbury and Windham, 
Vt., in Hot Springs, Ark., and in other places. 


CHAPTER IV. 


Prisms, Spherical and Cylindrical Lenses. 


The most simple glass used for spectacles is the Plane, 
both sides of which are parallel, forming a slab or true 
plate. Light passes through it without any refraction, 
provided it strikes the surface at right angles to its planes. 
As soon as we incline the slab so that the light falls on it 
obliquely, the light no longer follows its straight course 
but is bent more or less, according to the quality of the 
glass of which the slab is made. This inherent power of 
any glass to refract a ray of light in a certain proportion 
is called its index of refraction. Crown glass, for instance, 
has an index of about 1.5, and as air is taken as the unit 
(Chap. XIII), we express its formula by x f =*= f, the 
correctness of which may be demonstrated b}^ an experi 
ment illustrated in the following diagramm: 


p l 



M-' k e. 


The incident ray l is intercepted by the crown glass 
abed at i. When we raise the perpendicular p q and 
divide the distance between this and the incident ray l i 
at the height above the slab equal to its thickness into 
three parts and at e, the point of the exit of the ray, 
into two parts, then we have the direction the ray will 











PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 41 

take through the slab. At e it will again enter the air 
parallel to l i, but not in its exact prolongation which 
would be at m ; it is now by the refraction of the slab 
slightly shifted or displaced in the direction of e Jc. 

If we cut the above slab in the direction from b to c, 
we obtain two triangular wedge-shaped pieces of glass, 
called Prisms. In directing the side of one of these 
prisms, say a b c, perpendicularly to the incident ray, 
this ray will enter the glass in a straight line till it reaches 
the oblique side b c, when it will be bent in the same 
manner as the oblique ray was refracted in the slab. In 
a slab it is the oblique ray that produces the effect which 
in a prism is due to its oblique side. 



The ray l enters and penetrates the prism in a straight 
line until it touches i, where we erect the perpendicular 
p q upon b c, then we divide the distance between the 
incident ray and the perpendicular at q into two equal 
parts, when three parts of them from p downward (tow¬ 
ard the base) will show the direction the ray has taken 
outside the prism. If we place our eye at e, the object 
l appears at Jc, as if it was removed from i to d. 

When we hold the prism in such a position that the 
incident ray falls on it obliquely, the ray is first broken 
in the prism itself and then by the second inclined sur¬ 
face. 

The object at l appears now to be displaced outside 
the prism. 

Prisms have no focal power like spherical lenses, and 
cannot be measured by inches; their strength is simply 






42 HAND-BOOK FOR OPTICIANS. 



determined by the angle at b in degrees. Its opening at 
b confers upon the prism its strength and name. We 
have prisms of 1°, 2°, etc. The following figure repre¬ 
sents a prism of 45°, or the eighth part of a circle. 


A 



With a “trial box,” containing test prisms, the 
strength of a prism may be determined by neutralizing 
it by another; but to ascertain also the correctness of our 
test-prisms, it is necessary to construct a tool made of a 
protractor , like the following. 
















PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 43 


The joint B must be exactly in the center of the semi¬ 
circle DAE. That side of the riveted or stationary bar 
A B which is nearest E, is precisely 90° from either D or 
E. The arm B C is movable, and indicates the number 
of degrees of a prism placed in the opening ABC.* 
I present also another easy way of testing prisms by the 
use of a simple ruler. 

Take a ruler of 12", American measure; cut a notch 
in the edge at 6£ inches, large enough for the reception 
of the base of the prism. Place the longest part of the 
ruler on the line A B, and lay the base of the prism in 
the notch, so that the line A B is not broken in the 
prism; then see how far towards the right the line C D 
is displaced, and you will find that each degree repre¬ 
sents Ae of an inch; a prism of 16° displaces the line C 
D, therefore, exactly one inch. It is quite immaterial 

* Last year, the Geneva Optical Company introduced a new device 
for centering lenses and measuring the degrees of prisms. It is based on 
the principal of the above protractor, but is more practical and easily 
handled. They also introduced a novel lens measure which is based on 



the refractive power of flint glass. It accurately measures not only convex 
and concave lenses, but also cylindrical lenses and enables us to readily 
find the axis of the cylinder. This little instrument will become very popular 
among oculists and opticians who are not experts in measuring compound 
lenses by the regular analytical method. 


4 







44 


HAND-BOOK FOR OPTICIANS. 


how near or far we place our eye, as the deflection of 
the prism is not altered by it.* 

C 



The following diagram represents a prism of 4°; the 
dotted line shows the displacement of the object looked 
at through the prism of that strength, held six inches 
and a haif from the test-line. 

The thicker end of the prism is called the base; and 
“base in” means to place this end towards the nose-piece 

* This scheme is most skillfully brought into a scientific system by 
the invention of Chas. Pi Prentice. His prismometer is an ingenious in¬ 
strument; it does away with the heretofore crude method of numbering 
prisms by the angular deviation of their surfaces, in making use only of 
their refractive properties by numbering and measuring them in the 
metric system with great accuracy and uniformity. 







































PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 45 



of frame. In setting such glasses, care should be taken 
that a straight line is not broken in either of the lenses. 



If we take two prisms of the same strength, and lay 
them together so that the thick part of one covers the 
thin part of the other, we shall have a plane glass; one 
neutralizes the action of the other. The peculiar action 
of a prism consists simply in the displacement of an ob¬ 
ject seen through it. The object never appears where it 
really is; it is seen higher or lower, or more to the right 
or left than it should be. This is due to the different 
positions in which the prism is held. The dotted lines 
show the defective setting of one prism. 




















46 


HAND-BOOK FOR OPTICIANS. 


Spherical lenses, as we have seen in Chapter I, are 
ground by a segment of a sphere (globe or ball), im¬ 
parting to the lens the same curve, so that the finished 
lens itself is but a segment of a ball, and subject to the 
general laws of refraction. Any transparent sphere has 
its focal point just * at that part of its periphery which is 
opposite the entrance of the ray, and if we substitute for 
the ball one of its segments, which will be here a 
piano convex lens, the focal distance is not changed. If 
this ball is of 2" diameter, then the segment is of a 2-inch 
focus, or as we write it, to express the refractive power, 
+ But if we double this segment, then the focal 
point lies in the center of the ball, and its strength is one 
inch focus, or -f- -J-, equal to the unit of refraction in the 
inch system of numbering. 



This rule is good for lenses of any other number, be¬ 
cause the relative strength of a lens is constituted by the 
curve alone, and not by the thickness or thinness of the 
material of which it is composed. The two opposite 
curves can be widened by several plane glasses, put 
between them, without altering the focus, provided we do 
not change the place or position of the lens nearest the 
focus, and only widen the outside half of the double 
lens. For instance, take two + 8-inch periscopic lenses, 
put the hollow sides together, and measure this double 

* “This is strictly speaking only true when the refractive index is 1.5.” 

(Chas. F. Prentice.) 













PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 47 

lens; youthen have a lens of focus -j- 4-inch. Hold the 
inside lens steady and remove the outside lens ^ inch, 
and the focus is not visibly altered. 

When we hold a spherical lens vertically in our left 
hand, and, with our right lingers, turn it around its cen- * 
ter, without moving our left hand, we see no change in 
the object we are looking at. The movement around its 
center has no action on the object seen through the lens; 
it is the same as if the lens were held steady, and not 
moved at all. This should be remembered, because it is 
essentially different from the action of prisms and cylin¬ 
drical lenses. 

Manufacturers of optical instruments make use of this 
peculiarity in testing the correctness of lenses. They 
glue them upon a chuck of the turning lathe, and place a 
light at some distance in front. If the lens is well cen¬ 
tered, the light will appear in the lens perfectly steady 
when the lathe is set in motion; otherwise a glary circle 
will be visible in the lens. The larger the circle is, the 
more the lens is decentered; and it is only after its true 
center has been correctly determined, that the workman 
finishes the edges. All lenses of opera glasses, tele¬ 
scopes, etc., receive their finishing touch in this way on 
the lathe. 

We have already learned that prisms have the power 
of displacing an object without altering its size, and that 
their strength corresponds exactly with the degree of dis¬ 
placement of the object. With spherical lenses it is 
quite different; they enlarge the object when they are 
convex, and diminish it when concave, but leave the posi¬ 
tion of the object unaltered, except in special cases to 
be explained in the next chapter. 

If we consider a spherical lens to be but a concentra¬ 
tion of innumerable minute prisms, with either “base in” 
when convex, or “base out” when concave, and also bear 
in mind that in prisms the incident ray is refracted to¬ 
wards the base : then we shall at once understand why 
convex lenses unite or converge the rays to a plus or 
positive focus , and concave lenses diverge them to a 
minus of negative focus. 

Cylindrical lenses are ground and finished with a 


48 


HAND-BOOK FOR OPTICIANS. 


cylinder, instead of the segment of a ball used for grind¬ 
ing spherical lenses. When the outside of the cylinder 
is employed, the lens will be concavo-cylindrical; but 
when the concave side of a section of the hollow cylinder 
is used, the lens will be convex-cylindrical. The size of 
the cylinder imparts to the lens its strength; for instance, 
a cylinder of 5" diameter will produce a cylindrical lens 
of 5-inch focus, or of 8 D. Properly speaking, there is 
no common focus or focal point to a cylindrical lens, but 
instead thereof a focal line which is parallel to its so- 
called axis. We find here again an analogy to the pro¬ 
perties of prisms. While in spherical lenses the prisms 
encircle a common center or focus, they are in cylindri¬ 
cal lenses arrayed with their bases when convex, or with 
their apexes when concave, in a straight line across the 
lens, called the axis. We may say, therefore, that the 
fundamental forms of all lenses are but modified com¬ 
pounds of prisms; even the simple slab is a double prism, 
as we have seen at the beginning of this chapter. 



The axis of such a lens passes along the highest or 
lowest ridge of it, and is easily determined by moving 
the lens up and down, and finding by its gradual turning 
that line where there is no action at all. As long as the 
object seen through the lens moves with the motion of 
the lens, the axis is not yet found. 

The peculiar action of cylindrical lenses, producing an 
apparent lengthening or shortening of the object, alter¬ 
nately looked at through a convex and concave cylindri¬ 
cal lens, is best shown by the changed form of a square. 

A convex-cylindrical lens with axis vertical will 
lengthen the horizontal sides, producing a horizontal 
parallelogram. A concavo-cylindrical lens similarly 
placed, will have the opposite eifect, making the paral- 













PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 49 

lelogram vertical. This explains why a convex and con¬ 
cave cylindrical lens of the same power laid together, 



axis upon axis, will counteract each other, and restore 
the parallelogram to a perfect square. When we take 
two cylindrical lenses of the same strength, and place 
the axis of one vertically , and of the other horizontally , 
we destroy all cylindrical action, and retain only the 
strength of a simple spherical lens of the same num¬ 
ber or power as that of the cylindrical lenses. Take, for 
instance, two lenses of — 2c, lay. their axes crosswise, 
and you have — 2s, which is neutralized by + 2s. But, 
if we lay the two lenses (— 2c) axis upon axis, we double 
their power in getting a lens of — 4c. — My first exper¬ 
ience with these lenses was about twenty-five years ago; 
they were the piano-cylindrical, and their axis was set 
either in the vertical or horizontal meridian. I soon tried 
to combine the cylindrical with a spherical lens, gluing 
them together with Canada balsam, till I was advised 
from Berlin and Paris, that both corrections were ground 
upon the same lens. Of course, American opticians 
were promptly up to their profession, and since several 
years all combinations are ground here. Many blunders 
were made from all sides, and it seemed almost as if this 
Hew departure was a failure, till Nachet, and afterwards 
other opticians, improved the trial-frame. Yet, there 
was only a limited number of opticians who worked with 
the full understanding, and were able to convert one 
combination into another. As to my own researches, I 
must say that it took me a good while before I found the 
practical method of making this conversion with certainty. 

My simple process is based upon the above three fund¬ 
amental laws; let us bring them into such a shape that 
they will be remembered forever when once thoroughly 







50 


HAND-BOOK FOR OPTICIANS. 


understood. By means of these rules we can turn cross¬ 
cylinders into sphericals and compounds, and again into 
cross-cylinders without any trouble or error. 

I. Plus and minus cylinders of equal strength, and at 
same angles, neutralize: 

+ lc axis 90° Q — lc axis 90° = a plane. 

II. Two cylinders of equal strength and denomination, 
at same angles, double: 

— lc axis 180° Q — lc axis 180° = — 2c axis 180°. 

III. Crossed cylinders of equal value, produce sphericals: 
+ lc axis 90° 0+1° ax i s 180° — + 1*. 

The first two laws coincide with the properties of 
spherical lenses and need no further explanation, but the 
third law throws a new light upon sphericals; it demon¬ 
strates the fact that’spheres are only crossed cylinders, of 
which the proof is easily made. We know that + 2s is 
neutralized by— 2s ; we also know that the crossed cylinder 
— 2c axis 90° 0 — 2c axis 180°, = — 2s, 
and that the concave sphere (— 2s), or those concave 
cylinders combined, will turn + 2s into a plane or slab. 
Now, instead of placing both cylinders at once upon the 
convex lens, we may take the first one at 90° and lay it 
on the spherical lens. I wish, the reader would make 
here this experiment for himself, as it is of the greatest 
interest to notice practically the changes gradually brought 
about in the spherical lens by the addition, first of one 
and then of the other cylinder. As the lens — 2c axis 
90° represents only one half of the power necessary to 
neutralize the test-lens, its application to the spherical 
lens is quite singular, as it totally alters the nature of the 
sphere by turning its non-corrected half into a distinct 
convex cylinder axis 180°, which is fully neutralized only 
by the addition of the second concave cylinder at the 
same angle [180°]. 

There are two combinations of crossed cylinders: 

1. Both cylinders have equal signs, but one is stronger 
than the other. For instance: + 2c axis 90° Q + 
lc axis 180°; this crossed cylinder can be converted 
into the following compounds: 

+ 2s Q — lc axis 180°, or + Is Q -f lc axis 90°. 


PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 51 

2 . The cylinders have mixed signs, no matter how strong 
each cylinder. 

+ 2c axis 90° 3 — 2c axis 180°; their equivalents 
are: + 2s 3 — 4c axis 180°, or — 2s 3 + 4c axis 
90°. 

To come to the above answers, we have to turn one of 
the cylinders into a spherical lens, making use of the 
third law. For instance: + 2 c axis.90°, requires the 
cylinder -j- 2 c axis 180° to turn it into + 2s; but then we 
have to neutralize this first addition by — 2 c axis 180°, 
in order not to alter the original strength of the crossed 
cylinder, although we have changed entirely its form. 
Thus: 

+2caxis 9O°0+lcaxis 180°; by adding 

+ 2 c “ 180°3—2c “ 180°, which is a plane, we receive 

+ 2 ^ 3 —lc axis 180°, the first answer. 

We take the same crossed cylinder, and turn the second 
one into a sphere. 

+ lc axis 180° 3 + 2c axis 90°; we add 
+ lc “ 90° 3— lc “ 90°, = plane, and We get 

+ Is 3 + lc axis 90°, the second answer. 

Remember that each time we added something to the 
crossed cylinder in the problem, this something repre¬ 
sented only a plane or slab, according to the first law, 
although it was really a crossed cylinder in disguise; but 
it completely changed the nature of the test-lens, which 
a plane glass cannot do. 

Let us now see about the second combination with 
mixed signs, by making use'of the same process. 

+ 2c axis 90° 3 — 2c ax i s 180°. We again add 

+ 2 c “ 180° 3 — 2 c “ 180% ==. a plane, and get 

+ 2s 3 — 4c axis 180°, the first answer. 

We now turn the second cylinder into a sphere: 

— 2c axis 180° 3 + 2c axis 90°, adding 

— 2c “ 90° 3 + 2c “ 90°, = 0, we come to 

— 2s 3 + 4c axis 90°, the second answer. 

To turn a compound lens into a crossed cylinder, we 

make use of the same experiment of which I spoke at the 

beginning of this article. Let us take the last answer: 

© © 






52 


HAND-BOOK FOR OPTICIANS. 


— 2s 0 + 4c axis 90°, and add 

+ 2c axis 90° 0— 2c “ 90°, = 0, we get 

— 2c axis 180° 0 + 2c axis 90°, a crossed cylinder. 

The first cylinder of this answer [— 2c axis 180°], may 

puzzle the inexpert, but Law III explains it quickly. We 
can neutralize — 2s either by + 2s, or, as we did in the 
problem, by the crossed cylinders: 

+ 2c axis 90° 0 + 2c axis 180°. 

By laying + 2c ax. 90° on the test-lens, the remainder of 
it ought to be a cylinder which is neutralized by + 2c 
axis 180°, according to Law I. 

As I used in the foregoing problems the most simple 
values and always the same numbers, let us now select a 
problem with varied numerals as a further illustration. 
For instance: 

+ 1.75c axis 90° 0 — 2.50c axis 180°. To turn the 
first cylinder into a spherical, we must combine it with 
+ 1.75c axis 180°; then we have to add — 1.75c axis 
180°, in order to neutralize the first addition, thus: 

+ 1.75c axis 90° 0 — 2.50c axis 180° 

+ 1.75c “ 180° 0 — 1.75c “ 180° = plane 
+ 1.75^ 0 — 4.25c axis 180°, or 

+ 1.75c axis 90° 0 — 2.50c axis 180° 

+ 2.50c “■ 90® 0 —2.50c “ 90° 

+ 4.25c axis 90° 0 — 2.50s. 

I will direct your attention now to the important 
question: “ Is there any essential difference between, a 
crossed cylinder and its equivalent compound ?” Ap¬ 
parently there is a great difference, and yet there is none 
whatever; every compound lens is a crossed cylinder, 
and every crossed cylinder is a compound lens. Let us 
turn, for argument sake, the following crossed cylinder 
into a compound lens: 

+ 1.50c axis 90° 0 — 2 c axis 180°; by adding 
+ 1.50c “ 180° 0 — 1.50c “ 180° = 0, we get 
+ 1.50s 0 — 3.50c axis 180°. 

Now, let us turn the compound lens again into a 
crossed cylinder. In order to do this according to pre¬ 
vious rules, we substitute for the spherical part of that 








PRISMS, SPHERICAL AND CYLINDRICAL LENSES. 53 

lens (+ 1.50s), its equivalent, viz. + 1.50c axis 90° 0 
+ 1.50c axis 180°, thus: 

+ 1.50c ax. 90° O + 1.50c axis 180° 0 — 3.50c ax. 180°. 
This formula, as it stands, is nonsensical, because we have 
only two surfaces on a lens for cylindrical corrections; 
but by looking at it critically, we observe that -{- 150c 
axis 180° Q — 3.50 axis 180°, is only one cylinder of 
— 2c axis 180°; and the actual formula of that triple 
cylinder, therefore, is + 1.50c axis 90° 0 — 2c axis 180°, 
which is the crossed cylinder we first turned into a com¬ 
pound lens, and then again into its first form. 

This practical test evidently demonstrates the fact that 
two faulty meridians in the eye, when 90° apart, are cor¬ 
rected as well by a compound lens as by crossed cylinders. 
But as cross-cylinders require greater care in grinding 
and fitting, all competent oculists and opticians prefer 
to substitute compounds, as being less liable to mistakes, 
and accomplishing the same visual correction. 

About the year 1850, a French optician, Galland de 
Chevreux, introduced crossed cylinders instead of spher- 
icals, claiming that they not only corrected presbyopia, 
but also the small degree of astigmatism with which 
nearly every eye is afflicted. A careful comparison of 
them with spherical lenses will show the fallacy of his 
claim. This, and their high price, have brought them 
into disuse. “ In fact, it has been demonstrated, both by 
exhaustive mathematical calculation and experiments, 
that all crossed cylinders, for all deviations of their axes, 
may be replaced by sphero-cylindrical lenses”.* 

The use of cylindrical glasses has increased lately to 
such an extent that no optical establishment comes up to 
the requirements of the trade without being able to fill 
correctly the orders of oculists. One-tenth of all eyes 
are more or less astigmatic; and since oculists have taken 
the selection of spectacles in hand, the demand for 
cylindric glasses is very great. 

* “We farther are to suspect error in our estimation of the refraction 
of an eye seeming to demand cylinders combined under acute or obtuse 
angles.” Dioptric Formulas for Cylindrical Lenses, by Chas. F. Prentice, 
New York, 1888. 



CHAPTER Y. 


Optical Line and Center. 


About twenty-five years ago, a traveler for a New York 
manufacturing house, offered spectacles for sale which he 
called “ Perfected.” When I asked what he meant by , 
it, he said that the lenses were correctly set, the frames 
well tempered, and the whole spectacle perfect. To my 
great surprise, one lens of the first pair I examined was 
badly centered. He excused himself by saying that he 
was not an optician, he only represented the goods ac¬ 
cording to instructions given him by his employers, and 
promised that all goods I might order through him should 
be without fault. He admitted further that no member 
of his firm was a practical workman, but that the factory 
was superintended by a competent optician. Now, if this 
foreman really understood the meaning of an optical 
center in a lens, why did he not instruct the glass-setters 
how to be exact in the fitting of lenses to the frames, 
especially of those “Perfected Spectacles,” for which 
they charged $4.00 a dozen more than for other goods 
not stamped, but of the same style and quality ? I know 
not whether this name was invented only for the sake of 
extortion, or whether they charged so much more for the 
stamping of the temples, which was, indeed, nicely done 
in gold letters, and was something new at that time. 

To be able to readily determine the optical line of a 
lens, is more important for an optician than any other 
acquirement of his trade. It is the essential requisite 
for the correct manufacture of all optical instruments, 
— spectacles, opera glasses, telescopes, or microscopes; 
the optical center must have its right place and position, 
or the instrument will be incorrect and worthless. 

The best way to find this center is to look through the 
lens at a well-marked vertical line, drawn with pen and 




OPTICAL LINE AND CENTER. 


55 


ink and a ruler on a sheet of paper. Hang this paper 
against the wall some four or more feet from you; then 
take the lens between the thumb and first finger, extend 
your arm, shut one eye and look through the lens at that 
line. You will observe that the line is broken in the 
lens, and the more so, the nearer you move the lens 
towards its border. Figures a or b represent this phe¬ 
nomenon in concave lenses, and h or l as seen in convex 
lenses. 



Now, move the lens slowly towards the center till you 
find the line unbroken, as in figure c; mark this line with 
ink, and it will indicate the optic line of the lens in one 
direction. Then turn the lens 90°, so that the line on 
the paper and the mark on the lens form right angles. 
Proceed in the same way as before, and you will find, 
very often, that the optic center is not always in the 
middle of the lens, as we see in figure d. 

The two lines should cross each other in the middle of 
the lens, as they do in figure e. This test will do for 








56 


HAND-BOOK FOR OPTICIANS. 


spectacle lenses; but the test for scientific instruments 
is more elaborate, as we have seen in Chapter IV. 



This somewhat circumstantial way of finding the center 
of a lens can be shortened as suggested by Dr. H. Knapp 
in this manner: ‘ 4 Look through the Jens at two lines 
crossing each other at right angles; when the prolonga¬ 
tions of the lines beyond the lens are unbroken, the point 
of the lens through which we see the crossing of the lines 
is the optic center of the lens.” 

I would advise you now to take at randon^ a dozen 
spectacles from your stock in trade, amT^examine them 
as to the correctness of their optic centers. You will 
find that many of them, highly valued in the market on 
account of their trade-mark, are greatly incorrect and 
good for nothing. It will cease to be a matter of aston¬ 
ishment that some of your customers could not see with 
one pair of spectacles, and yet found others of the same 
number pleasant and satisfactory. 

To decenter lenses is an easy task for any one who un¬ 
derstands the nature of the optic center. As we have 
seen in the previous chapter, under the heading “Prisms,” 
that we have to set them either base in or base out , it is 
sometimes necessary, in order to overcome certain defects 
of vision, also to decenter spherical lenses, and cause 












OPTICAL LINE AND CENTEK. 


57 


them to act like weak prisms. To fill such an order cor¬ 
rectly, it is necessary to first locate the optic center on 
the lens, then put the zinc-pattern (Chapter YI) as much 
as possible to one border of the lens, and make a mark 
around the pattern. 

In convex lenses the border nearest the optic center is 
the base; and any prescription of base in or out is cor¬ 
rectly filled, if we place this part (jf) towards the nose 
or temple according to order. In concave lenses the 
base is at g, or just the opposite from that of convex 
lenses. The base and center of a convex lens always 
fall together, in concave lenses they are separated; the 
base is at the border of a lens, and the optic center, of 
course, in its middle. If, therefore, we have to decenter 
a concave lens, base in , the optic line will appear beyond 
the middle, nearer the temple, but not towards the nose, 
as is the case with convex lenses. 

In order to explain the object of decentering lenses, 
I draw your attention to the fact that the eyeball moves in 
any direction about a common center of rotation by The 
action of six muscles, four straight ones, the other two 
oblique. The first are called superior rectus, inferior 
rectus, external rectus and internal rectus; they move 
the eyeball either upward, downward, outward or inward, 
while the two oblique muscles with the assistance of 
the four recti, rotate the ball in every other direction. 
Most movements of the eyes are in the horizontal meri¬ 
dian, and are alternately produced by the contraction of 
the internal and external recti. These two muscles are 
constantly taxed, and it is no wonder when one or the 
other will fail to perform its duty satisfactorily. In 
distant vision the muscles of the eyeball are almost at 
rest, but close work compels the recti interni to converge 
in order to direct the visual angle of both eyes to a near 
point. This is the reason why the internal muscles 
sometimes weaken and need bracing up, which is done 
either by prisms, or in very slight degrees of insufficiency 
by decentered lenses. As soon as a prism with base 
inward is placed before the eye, the rays will strike it in 
a more parallel direction, as if they were coming from a 
distant object, thus allowing the recti interni to relax. 


CHAPTER YI. 


Setting of Spherical Lenses. 


We work blindfolded when we are unable to find the 
center of a lens, and it will be by mere chance, if our 
work is correct. Rough lenses are not always well cen¬ 
tered; if they were, we would have simply to cut the 
size we need from their middle, and there would be no 
mistake. Many of them will be found so much decen- 
tered as to be useless for sizes 0 and 1, and are fit only 
for sizes 3 and four, or they may be altogether worthless, 
except for lenses to be decentered. 

We may take notice here of the lenses called the “ in¬ 
terchangeable”.* ^ They range from No. 3, the ordinary 
size of spectacles (although smaller numbers are used for 
children spectacles), to No. 2, the size of eyeglasses, up 
to No. 1 and 0; even to coquille sizes, No. 00 and 000. 
Most manufacturers of spectacles adopted these standard 
sizes of lenses, to enable the retailer to exchange the 
lenses from one frame to another without altering them. 
A lens of No. 2 spectacles fits exactly the ordinary size 
of eyeglasses. If you order 1-eye spectacles and eye¬ 
glasses, you can exchange the lenses from the spectacles 
to the eyeglasses, or vice versa, as you like, they always 
fit. 

In a well-centered lens the edges are equally thick 
on their opposite borders, and a little practice will enable 

* The first impulse in this direction was given by Noel, who patented 
a frame without screws, the lenses to be sprung in like watchglasses. 
Albert Lorsch introduced these spectacle and eyeglass frames since 1869, 
together with finished lenses of a certain size, about eye 2, which fitted 
either frame, and which he called “Lenses for the Patent Accommodating 
Spectacle and Eye Glass.” But the term interchangeable lenses is of a later 
date, and came into general use since the Bausch & Lomb Opt. Co. ac¬ 
cepted the standard sizes of the American Opt. Co., although other manu¬ 
facturers still clung to their old sizes, till at present there is hardly any 
manufactory which will not take, and correctly fill, an order for all in¬ 
terchangeable sizes. 


/ 





SETTING OF SPHERICAL LENSES. 


59 


the eye to see at a glance, and without looking through 
it, whether the lens is decentered or not. This saves us 
a good deal of time, as the principal test is then quickly 
determined. But we should not rely altogether on the 
judgment of our eye in this regard, as it requires a good 
deal of practice to detect small differences in weak lenses. 
e Any workman with good tools can perform in a short 
time more and better work than others who shuffle about 
the whole day long, wasting time and material, for want 
of proper implements. The most useful tool in setting 
glasses is the model or pattern made of thin zinc. If 
you have not yet made use of it, prepare a set of the 
different sizes and shapes of spectacles, as they come into 
your hands for repairs, and mark them according to the 
different sizes of the eye. Make a hole exactly in the 
middle, partly for purposes to be spoken of in the next 
chapter, and partly to suspend them on the wall within 
convenient reach, well-assorted according to size and 
pattern. About three dozen will fully assort you, and 
will save you, in the course of years, an immense amount 
of trouble and time. 

Another important tool is the marker , an instrument 
like a lead-pencil, mounted at one end with a small dia¬ 
mond. The marker is used to make a scratch around 
your pattern, after it is placed correctly on the lens. It 
will not cut the glass as a glazier’s diamond, because it is 
intended only for scratching purposes, and is, therefore, 
very cheap. On heavy lenses it is best to mark both 
sides, to prevent the breaking of the lens inside the 
mark. 

The next tool for our purpose is the sliding-tongs, an 
instrument employed by watchmakers and jewelers, who 
call it the “ dog-nose sliding-tongs; ” it is also used by 
opticians to chip the lenses. 



I have found the largest size the best for almost all 
lenses; but very thin glasses, for instance, for lockets or 






60 


HAND-BOOK FOR OPTICIANS. 


watches, which we may occasionally be obliged to grind, 
can be chipped with common flat pliers. The apprentice 
should practice this chipping on pieces of window glass, 
before he attempts to shape a lens and spoil it, perhaps, 
by inexperience in handling the tool. 

The proper tool used in factories, for doing quick and 
good work in this respect, is represented by the following 
figure. 



workman who called it the 4 ‘English Shears.” As I 
never found them for sale, I returned to the use of the 
“dog-nose sliding-tongs,” which answer very well the 
purposes of a retailing optician. The pieces a and b are 
dove-tailed into the shears and can be renewed when used 
out. They are -fa" broad, with straight, flat surfaces. 
The rivet at a is rather loose, so that there is ample play 
for the shanks to move freely sideways. They are used 
in a similar way as the sliding-tongs. 

The proper way to handle this tool is the following: 
The tongs, held by the right hand, should be applied 
loosely to the lens, and worked as we do a pair of scis¬ 
sors, with the difference that at the same moment we 
close them, we also give the upper part of the tongs a 
slight inclination to the outside and downward. The 
lower nose is kept right on the mark by the middle finger 
of the left hand which holds the lens. This effectually 
prevents the lens from cracking inside the mark. The 
outside movement of the tongs throws the chips and 
glass-splinters from us, and thus saves the eyes from 
injury. But a fine glass-dust also rises from the lens, 
and is very pernicious to the lungs. Hold the lens, 














SETTING OF SPHERICAL LENSES. 


61 


therefore, nearly at arm’s length, and blow the dust off 
before you breathe. 

As a rule, we should move the tongs outward; but we 
may come to a place which will not break readily, even 
by applying greater force. In this case we can some¬ 
times accomplish our task with ease, and without the 
risk of spoiling the lens, by moving the tongs upwards, 
using the lower nose for the breaking, and the upper as 
a guide. This alternate turning up or down of the 
tongs should be well practiced by the apprentice. In 
regard to the forward or backward movement of the 
tongs, it is immaterial which way we proceed with glass, 
but as to pebbles it should be always the backward 
movement. As every crystal has a-cleavage-plane in its 
lateral axes, the forward movement may accidentally 
cause a splitting in the direction of this plane, which 
rarely happens with the backward movement of the tool. 

It is hardly necessary to mention that the stone has to 
be turned from you when grinding. I have seen only one 
jeweler (and he, too, styled himself “optician”), who 
turned the stone to him, as he had seen done by a street- 
grinder. Is it to be wondered that he complained after¬ 
wards of not being able to get a smooth edge on his 
glasses, or that they looked as if rats had given them the 
finishing touch ? 

I do not think it out of place to say here a few words 
in general about the grinding of lenses. Almost all 
manufactories grind them into a sharp bevel, which is in 
my opinion an unnecessary trouble, and, besides, shows 
very little sound judgment. The grooves of most frames 
are not pointed, but rounded off, whether they are made 
of soft material or metal; and the lens, to properly fill 
such a groove, should be also rounded off. This will 
have the double advantage of being less liable to crack, 
and less troublesome to finish. Sharp-pointed lenses 
easily split shell or rubber frames, when the latter con¬ 
tract in cold weather; or they themselves are chipped by 
metal frames, when they are tightly fitted. To overcome 
this difficulty, and to establish a practical method of 
fitting lenses to the frames, I will describe the method 
which I have adopted. After the lens has been well 


62 


HAND-BOOK FOR OPTICIANS. 


shaped and sufficiently reduced by the sliding-tongs, I 
grind off the sharp edge on one side by passing it quickly 
over the revolving grinding-stone. A few revolutions 
will accomplish this, and will give it a small but distinctly 
visible bevel. Then I do the same with the other side, 
by turning the lens alternately edgewise, to take away 
its unevenness. In less than one minute my lens has a 
finished appearance, and needs now only the final adjust¬ 
ment. The edges of the lens have then a rounded form, 
and when set in frames do not show any roughness, 
because the polished surface of the lens touches the 
border of the mounting, thus relieving me of the trouble 
to polish the bevel, which, however, cannot be avoided 
when the lenses are thicker than the frames, or when the 
grooves are very shallow. 

In regard to the present universally adopted habit of 
polishing the edges of lenses, I must confess that I do 
not approve of it, for the good reason that the reflected 
light from such bevelled surfaces is annoying to the 
eyes, and can be easily removed by giving them only a 
fine ground finish, which the Germans arid French call 
“matt.” Even frameless spectacles could be made in 
this way and would look equally stylish. But this re¬ 
form can only be effectually introduced by the unanimous 
co-operation of the oculists in rejecting in future all 
glasses with polished edges. 

The fitting of bevelled glasses into the groove of the 
frame is quickly done, and they are easily ground and 
shaped if they are of an oval or round pattern. Octagon 
glasses require more attention, especially when the frames 
are old and often repaired. The greatest care has to be 
taken with skeleton and grooved glasses, as the edge 
must be flat, and the bevel very small. The stone should 
be used till the lens is rightly shaped and the edge 
roughly flattened ; we should then finish the lens on 
emery paper Nos. 3 and 2, and lastly on No. 1 and 0 for 
polishing purposes. If the lens has to be grooved, No. 3 
is used only for the edge, but Nos. 2 and 1 for the bevel. 
It is better to finish the bevel before filing the groove, as 
a polished surface is less liable to chip in case the file 
should touch the edges. The grooving is always done 


SETTING OF SPHERICAL LEASES. 


63 


with a round file, never with a four or three-cornered 
one. The file will soon be smooth if used dry; it is 
therefore necessary to wet it constantly either with water, 
turpentine, benzine, or dilute sulphuric acid; the latter 
is most effective. But even these will generally ruin 
the file after the finish of one pair of lenses, thus con¬ 
siderably increasing the cost and labor. The best fluid 
for the preservation of the file and drill for our purposes 
is one that contains an access of camphor. Any mechanic 
knows that a new file should not be used at once for 
filing hard iron or steel, without passing it first several 
times over a soft material, as wood, brass or soft iron, to 
fill up the deeper parts of the file, giving strength to the 
exposed sharp points of it. Camphor renders the same 
service to our file used for grooving glasses, without 
interfering with its cutting qualities, if the fluid evapo- 
rates quickly enough to allow the camphor to clog up 
the deeper parts of the file. To do this by passing it over 
lead, would cause it to slip without cutting the glass. 
The formula for this fluid is : 


Spirits of Turpentine.1 ounce. 

Camphor Gum.1J “ 

Sulphuric Ether.3 drachms. 


The ether facilitates the solution of the camphor, but 
volatilizes so quickly that the file would be dry after a 
few strokes, if the turpentine did not retard its vola¬ 
tilization for a while. Keep the file, therefore, constantly 
wet while using it, and it will do service for a good 
length of time.* 

The drilling or boring of glasses for skeleton or 
frameless spectacles is done by a drilling machine; but 
if you have none, it can be done with a round file and 
the above fluid. Select a file almost of the size of the 
hole you need; break off the point, and commence the 
hole by moving to and fro the sharp edge of the file, 
previously dipped into the camphor preparation. Make 
a mark on the glass, then raise the file by degrees per¬ 
pendicularly to the lens, and use it as a drill by turning 
it slowly between the fingers. Each turn of the drill 

* Another excellent fluid for this purpose was lately introduced, called 
“Diamond Oil,” for drilling and filing in glass, porcelain, enamel, etc. 






64 


HAND-BOOK FOR OPTICIANS. 


must make the noise of a gnawing rat, otherwise the 
drill does not bite. When the hole is half through, com¬ 
mence on the other side, and reduce pressure gradually, 
to prevent a sudden advance of the file when nearly 
through. The holes are finished off by a three-cornered 
sinker, much larger than the hole itself, which bevels the 
edges of it, and prevents the breaking of the lens by the 
subsequent insertion of the screw.* 

There are many devices recommended to shape drills 
for glass-boring purposes ; all agree that they should 
never be pointed in the middle, but be rounded up, or be 
flat like a chisel. My favorite drills were always made 
of a round file (rat-tail), by grinding off two opposite 
sides, so that it had almost the shape of a square. Hold¬ 
ing the file at an angle of 60°, I smoothed the lower 
surface with the oil-stone, forming a slanting plane, and 
producing a sharp strong edge to cut with. Another 
good drill is made of a three-cornered file, sharpened in 
the usual way, but with one corner taken off, so that the 
cross section of the drill near the point is that of a trun¬ 
cated cone, and the end of the drill of a narrow chisel- 
shape. 

Not all files make good drills. Either they are not 
well tempered, or the grain of their steel lacks that pe¬ 
culiar cutting quality which we find in others. If you 
see, therefore, that your drill does not cut readily, throw 
it aside and try another file, till you find one that works 
well. I have often rehardened them, but generally with¬ 
out success; the steel was not precisely of that quality 
which is necessary to make a good drill. When you 
have secured such a file, take jealous care of it; I have 
used some for years, and found them always reliable 
like old trustworthy friends. 

* It is safer to stop drilling as soon as we reach an opening, no matter 
how small, because it is easy now to widen the hole with an ordinary 
broach, wetted with the above fluid, in the same way we enlarge a hole in 
a brass plate, provided the broach is not pressed or forced into the hole 
but moves loosely in it. 



CHAPTER VII. 


Measuring and Setting of Compound Lenses. 


Simple defects in the refraction of eyes can be cor¬ 
rected by spherical cx or cc N glasses; and when their 
right number or strength is selected for each eye sepa¬ 
rately, and afterwards correctly set in suitable frames, 
such spectacles will always give satisfaction. Nine-tenths 
of those in need of glasses are well suited with simple 
spherical lenses, and can be rightly served by the opti¬ 
cian as well as by the oculist, who, if he is nevertheless 
consulted by over-anxious people, can do no more than 
we do: he uses his test-types to find the extent of the 
error of refraction, and selects the spectacles accordingly. 
But others require something more than an optician is 
able to do; these should be sent to an oculist, who, after 
a professional examination,will give his orders, generally, 
for compound lenses. 

Compound lenses are combinations of spherical, pris- 
matical and cylindrical glasses, of which two, or in some 
cases all three, are ground on one and the same lens. 
The most simple combination is when the plane sides of 
a prism are ground into the spherical shape of either cx 
or cc, without altering the action of the prism. 




An order for such a lens will read for instance: 
+ 3 S O prism 2°, or perhaps:— 2 S Q prism 3°. 

The combinations of compound glasses are so mani¬ 
fold that they have'to be ground always to order, as no 
optician can have them in stock. We should never rely 
on the faithful execution of our orders by the grinder; 
for instance, we may have copied them indistinctly, etc. 
It is therefore advisable to remeasure all lenses before 







66 


HAND-BOOK FOR OPTICIANS. 


we fit them to a frame. Let us take the first lens as a 
test. We have here a spherical + lens combined with 
prism of 2°. I suppose that each of my readers has a 
trial-box with all the different lenses; if he has not, he 
should procure one as soon as possible, for no optician 
can do without it.* We first take from our box a prism 
of 2° and place the thick end upon the thin one of our 
lens. We will see at once that the optic line, which was 
before near the border, is now in the center. We then 
take — 3 S and place it on the other side of the lens; 
these three together must now be a plane, or the lens is 
not correct. 

The next combination is the sphere with a cylinder. 
One side of the lens is ground spherically, the other side, 
cylindrically. Such an order reads: + 2.5 S 0 -f 1.25 C. 
The test is the same as before. With— 1.25 C we neu¬ 
tralize the cylindrical action in this lens by laying the 
two axes so as to cover one another perfectly, and by 
adding — 2.5 S we must again have a plane lens. The 
grinder always marks the axes by little scratches upon 
the edges of the lenses, so that we have no trouble in 
measuring them. 

But how, when the grinder forgets to mark the lens, 
or when we are compelled to find the formula of com¬ 
pound spectacles, having no mark, handed us by a 
stranger to duplicate them ? 

Let us first see if they are decentered; if so, we will 
find one side of the lens thicker than the other, or that a 
horizontal line is elevated or lowered by turning the lens 
between the fingers to the right or left. When this is 
the case, they are combined with a prism. We neutral¬ 
ize this prismatic action by trying different degrees of 
prisms till we get the optical line in the middle, or till 
there is no more breaking of the horizontal line. By 
turning the lens now, the optic line will not move up or 
down, as it did before the prismatic action was neutral¬ 
ized. 

Keeping these two lenses in position, we notice whether 

* They were formerly imported from Europe, and Natchet’s Trial 
Boxes were considered the best; but at present, we manufacture them" 
equally as good and cheaper in America. 



! 


MEASURING AND SETTING OF COMPOUND LENSES. 67 

the cyl. part of the lens is cx or cc, by moving it to the 
right or left in front of a vertical line. When this line 
follows our motion, the lens is concave, and has to be 
defined by a cx cyl. lens. The remainder of our lens is 
simply spherical, and easily measured. To prove the 
measurement, and especially to determine the right posi¬ 
tion of the angle of the cyl. part, we first neutralize the 
prism, and then the sphere, and lastly find the axis of 
the cyl. part by following the rule given in Chapter IV. 
We now mark this line with pen and ink, and place our 

lens on the following figure. 

© © 



The center of the lens must be exactly over the center 
of the circle, and the horizontal line marked on the lens 
from nose to temple must cover the horizontal line AB. 
We now observe in what direction the ink-mark points, 
and we have the degree of the cyl. axis. In making 
this proof we must hold always the outside of the lens 
upwards, not towards the paper. 

All measurements of the physician refer to the position 
he takes toward the patient, his right is the patient’s left. 












68 


HAND-BOOK FOR OPTICIANS. 


I have made for my own use this delineation on strong 
paste-board, covered with white paper, and find it very 
handy and more accurate than anything I used before. 
The little lines are useful guides for finding the right 
position of the zinc-pattern, and dispense with the labor 
of searching for the true center through its hole. 

I have tried to explain this matter in as few words 
as possible, and in the most practical way; but some 
may think it too complicated a task, and lose confidence 
in their ability to overcome certain difficulties. Just try 
it, and if it takes a whole hour to measure a compound 
lens over and over again, you will laugh at this “Sphinx” 
afterwards, when you will be able to solve the problem 
in five minutes. 

It is absolutely necessary to know well how to meas¬ 
ure compound lenses before we are able to set them 
correctly. I admit that there are difficulties which will 
puzzle the inexpert, and will lure him into a different 
calculation altogether. I give here a few illustrations: 
We have, for instance, a lens — 0.50 S Q — 1 C ax 90°, 
the formula of which we do not know. By looking 
through the lens we see at once that the concavity is in 
excess of the convexity, if there is any at all. We first 
look at a vertical line, and notice whether it will follow; 
if it does, and we pursue our investigation and correct 
the cyl. action by a suitable cx cyl. lens, we are on the 
right track. But if, perchance, we had turned the lens 
i of its circumference, and had examined it in this posi¬ 
tion, which is at right angles to the true cyl. axis, we 
find that the vertical line does not follow, but acts as a 
cx cyl. lens, and we have to make the correction in this 
case with a concavo-cyl. lens. The consequence will be 
that we make cross-cylinders, and adding — 1 S to the 
— 0.50 S, which is the real amount of its spherical con¬ 
cavity, the formula found will be — 1.50 S 3 1 C 

ax 180°, which is the periscopic equivalent of the first 
biconcave lens. 

For another test let us take — 0.50SQ + 2Cax 
180°. The cyl. axis is easily detected in such a lens by 
its shape; but, for argument’s sake, I suppose we have 
fallen into the same error, and again produced cross- 


MEASURING AND SETTING OF COMPOUND LENSES. 69 

cylinders, thus turning + 2 C into -f 2 S. Then we have 
to add + 2 S to — 0.50 S, which would give + 1.50 S, 
and our formula would be~+ 1.50 S Q — 2 C ax 90°, 
the equivalent of the test-lens. 

We take another lens: + 1.75 SQ + 0.50 C ax 90°, 
and by the same process we will get -f- 2.25 S Q — 
0.50 C ax 180°. In order to find the formula, the axis 
should always be marked by small scratches at the border 
of the lens, or by pen and ink on a lens already fitted. 

The fitting of a compound lens to a frame next lays 
claim to all our attention, if we will do justice to the 
general rule, i. e., to bring the spherical center before 
the pupil of the eye. When we have marked the cyl. 
axis in ink across the whole lens, and have neutralized 
the cyl. action by its opposite, we must next observe 
where the optic line crosses the lens 90° from the cyl. 
axis and mark it likewise in ink, but in a manner differ¬ 
ent from the line of the cyl. axis, say by little dots. 
We now lay that point of our lens where the two lines 
cross, exactly over the center of the test-figure, turn the 
axis of the cyl. to the prescribed angle, and mark by 
little scratches at the edges of our lens where, it touches 
the horizontal line A B. These marks are guides to 
direct us in regard to the nose-piece and temple. We 
must take care that the hollow side of the lens lies upon 
the paper, because that side will be towards the eye. 
Our zinc-pattern, after which we mark the lens, must 
have a hole exactly in the middle, and a marked line 
from the nose-piece to the temple. 



Through the hole we can see the point where our ink- 
marks cross; we put the line of the pattern so that its 
continuation strikes the scratches made before as a guide 
for the nose and temple, and after ascertaining once 
more by careful examination that everything is right, we 
run our marker around the pattern. Before chipping off 






70 


HAND-BOOK FOR OPTICIANS. 


the superfluous part of the lens, we take a small wooden 
ruler, place it on the lens, touching the two marks for 
nose and temple, and make two other fine scratches 
inside the mark just made for the size of the lens, long 
enough not to be ground away in the finishing process. 
After the chipping, we have only to pay attention that 
our lens retains a nice oval shape, and that the edges 
are well bevelled v 

Any optician who follows these instructions cannot 
fail to give full satisfaction to the most exacting oculists, 
no matter at what angle the axis of the lens has to be 
placed. I believe that some opticians are careless in 
marking the true center of the lens, and, to find the angle, 
use designs similar to the following: 

O O 



I republish this cut as a sample of the incorrect man¬ 
ner in which they are generally made, and to guard 
against their use by any optician. 

In the “ 4 Jewelers ’ Circular and Horological Review” 
of November, 1885, I find on page 312, the strange com¬ 
plaint of a well-known oculist, saying: “ You will seldom 
find a workman who can exactly set a cylindrical lens at 
the axis required, unless the axis named be 180° or 90°. 
You will probably have to tilt the frame a little, either 
up or down, to obtain the exact position required. That 
they set more lenses wrong than right, has been my 
experience.” 

If his opticians use the above design to find the angle, 
their lenses must, of course, be incorrectly set, except 
in those two directions, as they are the only correct ones. 
It is, therefore, no wonder that the “Doctor” finds 
fault with his optician. 




CHAPTER VIII. 


Selection of Spectacles. 


This chapter is written for young opticians and such 
persons as have not yet acquired sufficient experience in 
the selection of spectacles, to overcome the many vexa¬ 
tions incident to their particular trade. 

I refer first to the pupil distance , as this is the main 
point of a good fit of spectacles. Pupil distance is the 
length between the two pupils, measured from the 
middle of one to the middle of the other. This distance 
is never smaller than two, nor larger than three inches. 
The eyes of little children, as well as those of the largest 
men, are within this compass. The average pupil dis¬ 
tance of a grown person is 2§ or 2J inches, and these are 
the standard sizes the manufactories use for most spec¬ 
tacles they make. But an optician is obliged to have for 
any emergency an assortment of all the different widths 
ranging from 2 to 3 inches. Children require 2" and 
2£"; boys and young girls 2^" and 2f"; grown persons 
with small faces use mostly 2§". A full face needs 2£" 
and 2f". Near-sighted people have generally a large 
pupil distance, and very often require as high as 2f 
inches. I have had in my extensive practice only three 
customers whose pupil distance reached fully 3", and all 
of them were near-sighted. I myself use 2£", and share 
the same fate; I, too, am near-sighted. 

An ordinary dealer has a fair assortment with spec¬ 
tacles or frames from 2to 2§", most of them of 2f" 
and 2J" pupil distances. 

A simple way of finding the size of spectacles is to 
measure the length of the nose-piece and one eye , which 
gives exactly the true pupil distance: 




72 


HAND-BOOK FOR OPTICIANS. 


a 


4 



because if you shift the line a b to the left, so that a is 
vertical to a', then b will be vertical to b\ which are the 
true centers of the frame. 

Another important point is the selection of a proper 
nose-piece. People with a low or shallow bridge should 
not, or rather cannot wear eyeglasses; and even spec¬ 
tacles of the ordinary size are not satisfactory, if the 
nose-piece is not shaped so as to correct the deformity of 
the nose. Formerly there were only three nose-pieces 
in use, the C, K and X, to which lately have been added 
the snake and saddle nose-pieces. 



Snake. Saddle. 

The X and snake nose-pieces are the best for low 
noses and street glasses; the last is especially useful in 
removing the glasses far enough from the eyes to save 
the eyelashes from coming in contact with them. The 
only objection to most of these nose-pieces is that they 
are rather thin, and consequently cut the nose, if the 
skin is tender, as is the case with children and ladies. 
Instead of making the nose-piece broader, Dr. Hubbel 
invented a nose-guard, a broad attachment to the nose- 
piece, which, of course, prevents the cutting of the nose, 
but at the expense of its look. 








SELECTION OF SPECTACLES. 


73 



A decided improvement in this respect was introduced 
by the Eggleston & Sibley’s “adjustable cork-noses for 
spectacles.” They can be easily applied, look well, 
protect the nasal crest, and enable us to correct the posi¬ 
tion of spectacles. 



If the nose-pieces were made sufficiently broad, well¬ 
shaped and polished, they would not need any lining of 
turtle-shell or cork, such as have been formerly made at 
some additional expense, while the broader nose-pieces, 
answering the same purpose, can be manufactured at 
almost the same cost as the thin ones now in use.* 

Heading spectacles should always be in such a position 
as to permit us to see through the middle of the glasses, 
without being obliged to bend our head down or forward. 
We should be able to see at an angle of 45° through the 
middle of the glasses with our head straight, and by 
merely lowering our eyes in that direction. These glasses 
must be placed considerably lower than the street 
glasses, which, on the contrary, enable us to see through 
the middle of the lens when looking straight ahead. 
The military rule for this position is that the eyes should 
strike the ground at forty steps from us, which is about 
one hundred feet. Near-sighted persons should be fitted 
in this way. It looks very bad when the street glasses 
sit too low, and oblige the wearer, in order to see through 
the glasses, to throw his head back, as if he were star- 
g azi ttg- 

* Since the publication of the first edition, some manufacturers of spec¬ 
tacles have introduced the swelled nose, by widening unduly the middle 
part of the bridge at the expense of the connecting shanks with the eye 
wire. Never have I seen so many broken nose-pieces as lately. 








74 


HAND-BOOK FOE OPTICIANS. 


In regard to this stooping position for working pur¬ 
poses, I may mention here the reason why people should 
not bend their head forward, but keep it erect while 
reading, etc. Any medical book will inform you that 
the arteries which carry the blood from the heart to the 
head and to other parts of the body, are situated far 
beneath the surface, and that those blood-vessels which 
you can see just below the skin are veins which conduct 
the blood back to the heart, Now feel the muscles of 
your neck when erect, and again when stooping; they 
are soft and pliable in the first position, and hard and 
stiff when you bend your head forward. The arteries 
being situated below the muscles, their action is not 
influenced by the changes of the latter from the relaxed 
to the contracted condition; but the circulation in the 
yeins is considerably interrupted by their being com¬ 
pressed to a much smaller size than before. What is the 
consequence ? The pumping of the blood into the head 
goes on uninterruptedly; but the flowing off to the 
heart is obstructed, and we sooner or later suffer from 
headache, get dizzy, and have to stop work. How many 
times is a spectacle-dealer puzzled by such complaints of 
his customers, not knowing how to correct it ? At last 
these people by chance find among the cheap, common 
spectacles a better fitting frame, and, of course, tempo¬ 
rary relief. We not only lose this ill-pleased customer, 
but drive him to the conviction that twenty-five cent 
spectacles are just as good or better than two-dollar ones. 
We are the cause of his at length ruining his eyes by 
the use of these common spectacles, through our igno¬ 
rance of the nature of his reasonable complaints. Direct, 
therefore, great attention to the correct fitting of frames. 

The temples of the spectacles should, as a rule, rest on 
the ears. If one ear of a customer is a little higher than 
the other, which is not so uncommon as people think, 
we have to bend or tilt the temple for the higher ear up, 
and the other one down, else the pupil will not be oppo¬ 
site the center of the lens. In some special cases we 
may be compelled to incline both temples in order to 
bring the glasses into such a position that the lower part 
of them is almost touching the cheeks, and the wearer is 


SELECTION OF SPECTACLES. 


75 


able to hold his work near the body. This may be ac¬ 
complished also by directing customers to place the 
temples one or two inches above the ears. 

It is absolutely necessary to examine both eyes sepa¬ 
rately, and to correct each error of refraction by the 
proper lens. But there are cases beyond the sphere of 
opticians > i. e., when it is impossible to make the right 
diagnosis without preparing the eye for such an examina¬ 
tion. These patients should be turned over to an oculist; 
it would be an act of ‘ 4 charlatanry ” on our part to 
pietend to do full justice to such cases. Confine your 
skill to the limits of your trade, and you will be con¬ 
vinced that it requires all your knowledge, intelligence 
and energy to fill the place of an expert optician. Over- 
ambitious young men may commit the error of trying 
to combine the two branches of an oculist and an optician 
as far as spectacles are concerned ; but is it not the 
mistake of a builder who would be his own architect, the 
apothecary his own doctor ? The public in general fares 
better if these branches are divided, and ably represented 
by competent specialists: on one side the scientific oculist, 
on the other side the skillful optician, both experts in 
their particular branches. If we play oculist, why 
should not the oculist play optician, and keep a stock of 
spectacles on hand ? Therefore my advice: “ Suum 
cuique .” 


CHAPTER IX. 


Double Focus Single and Split Glasses. 


The failing of our eyesight manifests itself by the 
gradual lengthening of the focal distance. At first we 
see well at 14", then we are compelled to hold our book 
or paper at 15"; afterwards at 16", etc.; and the pro¬ 
gress of the lengthening of our focal distance, slow at 
first, soon takes a wonderfully rapid stride, if we hesitate 
to substitute by spectacles that part of our power of 
vision which is irrevocably lost. We are reminded here 
of the common adage: “One stitch in time saves nine” ; 
i. e., the early use of spectacles, when their assistance 
is necessary, saves our eyesight from its otherwise rapid 
failure. Nine persons out of ten, who come for their 
first glasses, confess that they have put off the use of 
them as long as possible, but have to yield at last. They 
are not aware of the great blunder they made by taxing 
their diminished power of vision in the same degree as 
they did when their eyes were enjoying their full 
strength. 

A well known fact of our “losing sight” is the im¬ 
provement of distant vision; the sight is going away 
from us, we gain at the distance what we lose near by. 
Let us see fifteen years later what has become of our 
customer’s eyes and his spectacles. The gradual failing 
of his eyesight has compelled him to increase the strength 
of his reading glasses, till he uses now'+ 2.50 Diopters 
(or -f 16-inch focus) for near vision; but his far point 
has also removed, and he finds it impossible to distin¬ 
guish the features of the minister, or the faces of 
people in the street 50' away from him. He asks for 
street glasses. And here arises the question: Is it ad¬ 
visable to combine reading with distance glasses? The 
most rational way is to take separate glasses for each 




DOUBLE FOCUS SINGLE AND SPLIT GLASSES. 77 

purpose. Most people will follow our advice, and change 
theii glasses accordingly. But we have to deal also with 
nervous, quick-tempered and impatient customers, who 
grumble at the slowness of steam, and will have every¬ 
thing go by lightning. They imagine that they have no 
time to change their spectacles; and indeed some people 
have not. There is the accountant, whose entries in the 
ledger from the journal force him to look at items about 
2 ft. from him. He cannot keep his seat and accom¬ 
plish his task comfortably, if he has to jump up, and 
bend his body, and stretch his neck right or left, to 
check off and make a correct entry. There is the 
paying-teller, who must have a sharp eye on his money 
and the party receiving it. There is the engineer watch¬ 
ing his engine, and looking every now and then at the 
steam-gauge; the teacher, the minister, the orator, the 
clerk, the lawyer, and many other persons, who find it 
absolutely necessary to be enabled to see well at a glance 
far off and near by. Can we accommodate these people 
without injuring their eyes? We can to a certain extent 
with double focus spectacles, each glass adapted to its 
special purpose, the upper part for distant, the lower for 
near vision. These spectacles are called “Franklin 
glasses,” because Benjamin Franklin was the inventor 
of them.* 


* Through the kindness of Mr. F. W. McAllister, of Baltimore, I came 
in possession of a photograph of the following letter, written by Thomas 
Jefferson, to his great-grandfather : 

Washington, Nov. 12, 1806. 

Sir: —You have heretofore furnished me with spectacles, so reduced in 
size as to give facility to the looking over their top without moving them. 
This is a great convenience; but the reduction has not been sufficient to 
do it compleatly. I therefore send you a drawing No. 1, so much reduced 
in breadth as to give this convenience compleatly, and yet leave field 
enough for any purpose. And I will thank you for a pair of spring frames 
made accurately to the drawing, and a set of glasses as mentioned in the 
same paper. 

Those who are obliged to use spectacles know what a convenience it 
would be to have different magnifiers in the same frame. Dr. Franklin 
tried this by semicircular glasses joined horizontally, the upper and lower 
semicircles of different powers, which he told me answered perfectly. I 
wish to try it, and therefore send you a drawing No. 2, agreeably to which, 
exactly, I will ask another pair of spring frames to be made, and a corn- 
pleat set of semicircular glasses as mentioned in the paper. These will of 
necessity give up in part the other convenience of loo kin g over them. 
With these glasses I will pray you to send me a pair of goggles with clear 
glass, and a little case of three magnifiers of different powers, shutting up 



78 


HAND-BOOK FOR OPTICIANS. 

There are two kinds in use: the double focus single or 
pantoscopic lenses, where the upper part is ground off to 
a weaker focus, and the split glasses , where the distant 
and near lenses are cut through the middle (or optic line), 
and finished so that the split forms a straight line in the 
frame from temple to temple. The optic line in these 
glasses is, therefore, right on the split. The wearer, of 
course, is obliged to look below or above that very line 
where the eye is most at ease, and where it feels comfort¬ 
able, according to facts demonstrated in Chapter Y. 

The double focus single lens has a serious defect. It 
confuses the wearer in regard to the true position of things. 
If we look at a straight horizontal line first through one 
part of the lens, and in the next moment, by moving the 
lens, through the other part, we observe that the line is 
considerably displaced. It is elevated or lowered as we 

in a single horn case. They are used chiefly for reading off the fine divi¬ 
sions of astronomical or geometrical instruments, and are commonly to be 
had in the shops. I presume these articles placed between two paste¬ 
boards may come safely by post. The amount shall be remitted you as 
soon as known. Accept my salutations and best wishes. 

Th. Jefferson. 

Mr. John McAlister, 

Chestnut Street, 48, 

Philadelphia. 

Eye glass, long diameter § I. 

No. 1. short diameter § I. 


from center to center of eye glasses 2| I. 



A compleat set of glasses from the youngest to the oldest to fit the 


frame. Silver frames. 

Eye glass, long diameter § I. 
No. 2. radius § I. 

from center to center of eye glass 2| I. 



Each eye glass is composed of 2 semicircular lenses, the lower of a 
greater magnifying power than the upper, that is to say, of the next No. 
to the upper one. 

A compleat set of half glasses to be sent, from the magnifier adapted to 
the first use of spectacles, to that suiting the oldest eyes, all fitting exactly 
the frames. Silver frames. 























DOUBLE FOCUS SINGLE AND SPLIT GLASSES. 


79 


look at it alternately through the upper and lower part. 
Both parts of the lens act as prisms, bases in the middle. 



If the dotted line in the single lens is the true position 
of an object, we see it through the upper lens at a, and 
through the lower at b , but never where it really is. 
People who wear such glasses may, by looking, while 
descending the stairs, through their lower part, reach 
the bottom sooner than they expected; and if they have 
not lost their spectacles by the accident, may stare in 
bewilderment through the upper part at the place whence 
they came so suddenly. 

Double focus spectacles never render the service which 
two spectacles will confer to the eye; therefore, we 
should not recommend them to people who have ample 
time to change their glasses according to necessity. The 
absence of the optic line in split glasses , and especially 
the prismatic action in double focus single lenses, both in 
opposition to the first requirements of good spectacles, 
will in the course of time undoubtedly injure the eye¬ 
sight. Their use should be limited to such persons who 
absolutely cannot do without them. We may compare 
their action with that of a water-proof cloak which will 
protect against the outside rain, but at the same time 
will retain the perspiration inside. This is combatting 
one evil by another one. 

In selecting such spectacles, we have to find the proper 
strength for each purpose. I give here the general rule 
for this combination: “Ascertain at first the strongest 










80 


HAND-BOOK FOR OPTICIANS. 



cx glass with which the patient can see best at 20/XX, 
then add for near vision plus-glasses, until he can read 
comfortably ordinary type at the normal distance, say 
14 inches.” You will find that the relation of distant 
and reading glasses are calculated by the following rule, 
in the inch measure: 

Multiply by 3 from + 16 to + 11 

“ 2 “ + 10 to + 6 

“ li “ 5 down. 

When people wear + 16, we may try -f- 48 or + 60 


“ + 11 , 
“ + 10 , 


+ 30 “,+ 
4 20 “ 4 


36 

24 


c i 


One of these numbers is generally correct. In all 
cases where people insist on having double focus glasses, 
we should persuade them to take split glasses. 

One of my customers who had at my recommendation 
commenced with split glasses, was annoyed by the con¬ 
stant remarks that his glasses were broken, ordered them 
replaced by double focus single lenses. In vain I gave 
him all the points in regard to their prismatic action, and 
finally advised him to be very careful, till he had accus¬ 
tomed himself to their use. 

About a month afterwards, he entered the store in 
great excitement, and asked me to replace the split 
glasses. He had just left the street car, and by stepping 
over the gutter had missed the opposite curb-stone 
(which he saw through the lower part of his glasses to 
be nearer to him than it really was). He would have 
probably broken a leg, if he had not luckily grasped a 
lamp-post, just in his reach, to prevent a serious fall. 

The prismatic action in double focus lenses can be en¬ 
tirely overcome by taking for distant vision a full lens 
of the necessary strength, and then cement on it, a little 






DOUBLE FOCUS SINGLE AND SPLIT GLASSES. 


81 


below the optical line, a small lens which in addition to 
the power of the distant glass will produce the reading 
glass. Such lenses were lately introduced by the Geneva 
Optical Company, as “Morck’s Perfection Bifocals.” 




CHAPTER X. 


Colored or Tinted Glasses. 


“The blind man is a poor man, 

And the poor a blind man is. 

The one, of course, can see no man ; 
The other—no man cares to see.” 

The terms, blind men and beggars , are almost synony¬ 
mous, and indicate the great misery attending the loss 
of sight. Fortunately we live in an age in which science 
has investimated the “evils that befall mankind,” and we 
can say with pride: the blind man shall see; not by a 
mysterious wonder, but by the scientific skill of experts. 

Among the modern appliances to relieve the sufferings 
of an afflicted eye, Protection Spectacles ," set with col¬ 
ored lenses, take a prominent place. They soften the 
excess of light, otherwise so annoying and hurtful. Since 
spectacles were invented, people have experimented with 
different colors, giving preference at one time to this, at 
another time to that color, according to fashion, entirely 
disregarding optical laws, till they have settled for the 
present, with scientific reasons, upon the tint of smoke. 
To comprehend this question thoroughly, we must direct 
our attention first to the theory of colors in general, and 
see what we understand by the term “spectrum.” 

When we speak in an optical sense of colors, we ex¬ 
clude, of course, the pigments used by painters, who 
include among them even black and white , which are no 
colors at all. Black is the absence of light, and con¬ 
sequently of colors; white is the undivided light, con¬ 
taining all colors so combined that the different tints totally 
disappear. White light is, therefore, called “colorless,” 
although it needs only to pass through a certain medium 
to be resolved into the brightest colors, as is seen in the 
rainbow, where the falling drops act as the decomposing 
agent. The rainbow is a fair specimen of the Solar 




COLORED OR TINTED GLASSES. 


83 


Spectrum , showing the seven spectral colors, red , orange , 
yellow , green , indigo and violet. To these can be 
added brown , outside of the red, and gray , outside of the 
violet. By means of a prism we are enabled to produce 
this spectrum to perfection, and to investigate the 
particular properties of each color separately. We thus 
find that red is least refracted. It forces its way forward 
like a heavy ball or shot, while violet is the most refract¬ 
ed, yielding readily to the obstruction it encounters in 
passing through the prism. Scientists have found that 
the waves of red are nearly twice as long as those of violet, 
and this accounts for the impetuousness of its ray, 
which almost overcomes the interference of the prism. 
The waves of the other colors become gradually shorter, 
up to violet; and as the smaller waves act more gently 
upon the tender tissues of the retina, we might guess 
with some probability that violet would be the softest 
color to the eye. This would be a gross error. There 
is a decided difference in the effects of colors on the eye. 
It is pleasant to look at the dark green of a meadow or 
the foliage of trees; but it is very trying to use green 
spectacles, because our eyes are then constantly under 
the influence of one particular color. In fact, no color 
is hurtful to the eye as an object to look at; but if a 
special color is used as the medium to look through, it 
always acts more or less injuriously in the proportion as 
the shade is lighter or darker. There is no exception to 
this rule. We must bear in mind that a healthy eye is 
able to endure the full force of the whole light, and that 
any division and exclusion of its essential components 
will act detrimentally, as would be the case in breathing 
only oxygen or nitrogen separately, when the mixture of 
both in a certain proportion is the vital condition of our 
existence. No separate color is, therefore, a proper 
substitute for white light, for which our eye is con¬ 
structed, and so well adapted as long as it is in a healthy 
condition. 

But when the eye is impaired, and cannot stand the 
full strength of light, should we not shut off some of the 
most hurtful parts of the spectrum, and allow only the 
softer colors to act upon the tender organ ? Does not 


84 


HAND-BOOK FOR OPTICIANS. 


the physician regulate the diet of his patient by depriving 
him of certain food ? Certainly, so it seems at first to 
any superficial observer, but even the most rigorous diet 
does not deprive the patient of any of the necessary 
elements of his nutriment; only quantity and form are 
modified. Of the fifteen elementary substances our body 
contains, the four most essential are oxygen, hydrogen, 
nitrogen and carbon. To eliminate from the diet of a 
patient one of these four elements, would not be more 
inational than to suppress one color of the spectrum in 
favor of another. Neither green, blue, nor violet can be 
substituted for the peculiar union of all colors producing 
white light. Any shifting of the finely balanced ingre¬ 
dients of white light will act fatiguingly or even per¬ 
niciously upon the eye. Those of my readers who un¬ 
derstand chemical formulae will readily see the point in 
question. The (old style) formula of sugar is expressed 
by C 12 H u O n . Now take two atoms, each, of hy¬ 
drogen and oxygen from the molecule, and we have 
vinegar C 12 H 9 O 9 . By a similar process, “ sweet ” 
light may be made disagreeable by smothering one or 
more colors of the spectrum, or rather by increasing the 
effect of one particular color at the expense of the 
others. 

V\ e have seen that the exclusion of particular colors 
of the spectrum does not answer our purpose. It re¬ 
mains, therefore, to decide what can be done to protect 
the suffering eye from the injurious effect of light, with¬ 
out interfering with the essential combination of the 
thermic, electric and magnetic qualities of the sun’s 
rays, which peculiar combination agrees exactly with the 
construction of the eye, as the milk of the mother agrees 
with the healthy development of her infant. The most 
rational method is to diminish the whole amount of the 
light by smoked glasses. These do not alter the propor¬ 
tion of the different colors, and produce no change in 
their vibrations. They only lessen the amount of lio*ht 
without disturbing the proportion of its elements. The 
whole spectrum is thus uniformly reduced, and nothin^ i s 
changed by smoked glasses but the strength of the ex¬ 
cessive light. 


COLORED OR TINTED GLASSES. 


85 


To show that no special color by itself will satisfy the 
eye, I remind the reader here of the well-known experi¬ 
ment of saturating the eye with one color by excluding 
the others, and observing how eagerly the eye absorbs 
the complementary color after the test-color is suddenly 
removed. The easiest way to make this experiment is to 
cut from colored paper round pieceg of the size of a sil¬ 
ver dollar. Lay one of these circles upon white paper, 
and look for half a minute steadily at it, the eyes six 
inches from the colored circle. By removing this quickly, 
and looking always at the same place, we will see dis¬ 
tinctly the complementary color. This experiment be¬ 
comes very interesting when we try it with one eye only, 
the other being shaded. When at the moment of the re¬ 
moval of the test-color the open eye is shaded, we per¬ 
ceive with the other the complementary color as plainly 
as if that very eye had been directly exposed to the test. 
If the circle was mt, we will see instead a green one, 
which color is complementary to red. A yellow circle will 
produce violet; blue produces orange; and green will 
show red. The eye* seeks to be relieved from the strain, 
and is,"therefore much in need of the missing colors. 
It takes, indeed, a good while before the eye recovers 
from the fatigue, and is again able to receive the white 
light without seeing colors. * This experiment was 
known for many years, but nobody has yet drawn that 

* This peculiarity of the eye explains the sorcalled “Harmony of 
Colors, ” which does not depend on the will, or caprice, or personal taste 
of an individual, but is based on unchangeable laws of nature. By har¬ 
mony of colors we understand colors placed side by side in such a manner 
that ttey do not injure the effect of each other, and be satiating and 
pleasant to the eye. This is accomplished by adding to one particular 
color its complementary color in a judicious proportion, so that the eye 
will rest with ease on such a combination. Those who are familiar with 
these laws can make such selections in fitting up apartments, in dressing, 
etc., so that with the greatest simplicity they are able to produce a more 
favorable effect than is possible with the most extravagant expenditure 
without this sense of harmony of colors. Ladies are particularly in need 
of a thorough understanding of these laws, for most of them in the selec¬ 
tion of their colored dresses, bonnets and trimmings produce sometimes 
the greatest discord in the composition of colors. Red and green belong 
together, so do blue and orange, or violet and greenish-yellow, or indigo 
and yellow, also white and black. If one of these colors are selected for a 
dress, the complementary color should be used for the trimmings, and 
only the right proportion in which they are employed will show the re¬ 
fined taste of the wearer. 



86 


HAND-BOOK FOR OPTICIANS. 


lesson from it which it so clearly teaches. * Medical 
books leave the selection of a special color an open 
question, and permit the patient to choose for himself, 
or they are prejudices in favor of one particular color, 
as the celebrated Dr. Graefe was towards blue glasses, 
rejecting smoked almost entirely. 

It is needless to waste words further in regard to 
green, blue or violet spectacles, still manufactured and 
sold extensively to persons who are always on the look¬ 
out for something different from what others sell, and 
which are recommended the higher, the less such “ opti¬ 
cians” know of the science of their trade. The great 
trouble is that the manufacture of colored lenses is not 
scientific. There are thousands of different shades, due 
to the careless lyay in which glass is made. If competent 
glass-manufacturers would take it in hand to produce a 
clear colored crown glass, and would publish their 
formula, after their glass has been approved by leading 
oculists and opticians, all colored glasses could be 
limited to one dozen different shades, classified with the 
same certainty as we define now the white lenses by 
diopters. As colored lenses have only the object of 
softening the excessive light, it is rational to imitate the 
common practical way of shutting off the light by closing, 
according to necessity, the blinds of our windows or 
turning down our lamps. This is done by smoked lenses 
in their diiferent shades. There is hardly any exception 
in all the many defects of diseased eyes where smoke 
would not do all services expected from colored spec¬ 
tacles.. Even healthy eyes are in need of them in 
countries covered with snow, or where the intense glare 
of a tropical sun affects them. The Esquimaux make 

■ * Let me relat e another curious optical experiment which may serve to 

show the principle of the stereoscope. If we cut out of black paper two 
similar figures—two crosses, for example—and place them, their extremi¬ 
ties 'almost touching, at about three inches from the eyes, before a sheet 
Oi. white paper, we shall see three crosses, the middle one being dark and 
completely separate. This phenomenon-is explained by the simultaneous 
vision of the two eyes, and it is easy to show this by looking at the obiects 
successively with one eye. The experiment becomes still more interesting 
when, instead of black figures, we employ complementary colors—red and 
gieen for example. In this case we must use a dark background, and 
there will appear a white cross in the middle. 



COLORED OR TINTED GLASSES. 


87 


from wood a kind of coquille spectacles with a slit in 
the middle, to allow only a limited quantity of light to 
enter the eyes, and protect them from the dangerous 
effect of the strongly reflected sunlight which otherwise 
may cause snow blindness. 

Smoked lenses are absolutely necessary when the eye 
is inflamed, after most operations, and in other cases 
decided upon by oculists.* 

The study of the nature of colors and their combinations is very 
mteiesting, and should not be neglected by the aspiring optical student. 
How useful such knowledge may prove sometimes is shown by the follow¬ 
ing anecdote: 

“In a large factory one workman, in wielding his hammer, carelessly 
allow ed it to slip from his hand. It flew half way across the room, and 
struck a fellow workman in the left eye. The man was given in charge of 
an eminent oculist who after a careful examination stated that the eye 
was not injured, although the man averred that his eye was blinded by the 
blow. He brought a suit in the courts for compensation for the loss of 
half of his eyesight, and refused all offers of compromise. Under the law, 
the.owner of the factory was responsible for an injury resulting from an 
accident oi this kind. The day of the trial arrived, and in open court the 
oculist, who was summoned by the defense as witness, gave his opinion 
that the left eye .was as good as the right one. Upon the plaintiff’s pro¬ 
test of his inability to see with his left eye, the oculist satisfied the court 
and jury of the falsity of his claim. And how do you suppose he did it? 
Why, simply by knowing that green looked at through a red glass appears 
black. He had prepared a black card on which a few words were written 
with, green ink. Then the plaintiff was handed a pair of spectacles with 
different glasses, the one for the right eye being red and the one for the 
left eye green. The card was handed to him, and he was ordered to read 
the writing on it. This he did without hesitation, and the cheat was at 
once exposed. The sound right eye, fitted with the red glass, was unable 
to distinguish the green writing on the black surface of the card, while 
the left eye, which he pretended to be sightless, was the one with which 
the reading had to be done. ” 



CHAPTER XL 


Redressing of Spectacle Frames. 


Good spectacles require not only faultless lenses, but 
also properly fitted frames to render all the services we 
expect from them. The frames especially should be of 
the right size, neither too narrow nor too wide, and the 
nose-pieces be so shaped that, in street glasses, the pupil 
is exactly opposite the center of the lens. Reading 
glasses require a lower position in order to enable the 
wearer, by sinking his eyes for close work, to see also 
through the middle of the lens without bending his head. 
Accidents and careless handling will bring sometimes 
spectacles out of shape, and we are daily requested to 
redress them. The first attempt I made in this respect 
was directed only to the temples, which looked to be 
straight when open, but were pointing in different direc¬ 
tions when closed, often so much that I hardly-could 
replace them into the case. I have found very few 
jewelers who could properly redress spectacles, and I 
think it, therefore, necessary to devote the following 
lines for their instruction. The whole manipulation 
looks to be so simple, but I must say, it took me some 
years before I found the key for it. 

In order to save time and trouble, we should invariably 
commence with the nose-piece in connection with that 
eye which is the nearest correct. We should then bend 
the other eye so that both form a perfect plane, or that 
they stand in a straight line. Beginners do well to 
provide themselves with a small ruler about four inches 
in length, and use it as a test by placing it flatly on one 
eye, observing whether the other one is in the same plane. 
Then put it edgewise over the middle of one eye, from 
temple to nose-piece, and see whether the other glass is 
not out of line. When the middle part is corrected we 
examine the temples, and straighten them without pay- 




REPRESSING OF SPECTACLE FRAMES. 


89 


ing any attention to the position they will have in relation 
to the center part. If one of them extends too far to 
the outside, we should loosen the screw, or better, 
take out the glass altogether, and bend the joint upward, 
thus bringing the temple to a right angle with the center. 
It remains now only to give the finishing touch to the 
temples. If one•• of them stands lower than the other, 
the lens on that side will be raised to the greatest dis¬ 
turbance of the vision. To correct this, we close both 
temples, and see which one points exactly to the oposite 
joint; we take this as the model, by which we correct 
the other one. We cannot do this by bending the temple 
itself up or down, for this would undo a former correc¬ 
tion, which consisted in straightening the temples “ with¬ 
out paying attention to their position.” A little reflec¬ 
tion soon convinces us that the fault is not with the 
temple, but with the joint. In order to bend the joint, 
we must take good hold of it with some blunt cutting 
pliers nearest the eye, leaving almost the whole length 
of the joint at our disposal, and by means of strong flat 
pliers we can bring the joint to its proper position 
without the risk of breaking it. Any bending of spec¬ 
tacles should be done with two pliers, one in each hand. 
In addition to the above, we also need round pliers, 
especially in redressing the nose-piece. To ascertain 
finally the correctness of our work, we lay them edgewise 
upon a flat surface, for instance, on the show-case, and 
see, if the ends of both temples touch the glass; if it 
does, not, we have to go once more over the whole of the 
aforesaid manipulations. In case one temple has the riofit 
position when shut , but points side-wise and in a different 
direction to the other temple when open ,the fault is then with 
the joint-pin which is not in the same plane with the lenses. 
First remove the joint-pin, but not the temple, then in¬ 
sert a broach, and you will find that the planes of the 
lens and of the broach differ a good deal, showing 
at the same time in what direction you have to 
open the hole till broach and lens are level. Before you 
withdraw the broach, open the temple and you will see 
that it now points in the right direction, and will keep it 
after the insertion of a new joint-pin. 


CHAPTER XII. 


Use of Test-Types. 


To test vision with different kinds of print, as found 
in newspapers, etc., was practiced by spectacle-dealers, 
opticians and oculists up to recent date, and would be the 
style to-day if not at last the medical profession had 
taken the matter in hand, and initiated a new era by in¬ 
troducing a rational system. The first noticeable effort 
was made by Professor E. Jaeger, of Vienna, Austria, 
in the year 1854, in graduating types from the smallest 
to the size of posters, in different languages. The 
advantage of them over the old style was the systematic 
increase of the size of letters. But no direction was 
given how to use them, at what distance from the eye 
each of them had to be read, or what proportion of our 
eyesight was represented by them. The only advantage 
over the old, crude manner was that the letters were clear, 
and the paper white, and that thinking opticians soon 
acquired by practice, what number of glasses they had 
to furnish their customers who could read a certain size 
of print at the usual reading distance. But after all, it 
was nothing but guess-work, there was no law, no prin¬ 
ciple, no science in it; they served only to test the eyes 
at close distance, for reading, sewing, etc., and were of 
no service to test the eyes for distant vision. 

This problem was solved by Dr. H. Snellen, of 
Utrecht, Holland, in 1868, who determined the acute¬ 
ness of vision to the visual angle of one minute (1'), 
instead of forty seconds (40"), as was the general rule 
up to that time. In Chap. XXIII, “Range of Vision,” 
I based the calculation upon the old rule , that objects still 
could be seen when their visual angle was not smaller 
than forty seconds; this would enable us to distinguish 
objects at a distance of 5000 times its diameter (or more 



USE OF TEST-TYPES. 


91 


correctly 5156 times). But I think Snellen’s suggestion 
is more correct as regards the application of this rule 
to practical use. An object of one foot in diameter can, 
therefore, be seen only at a distance of 3437 feet, instead 
of 5000', as stated in that article. 

Perhaps some readers do not fully understand the 
meaning of a visual angle or “ angle of vision,” and do 
not know how to find, in common measure, the length 
of that part of the periphery which represents a given 
angle. A short explanation will, therefore, not be out 
of place. We know that each circle, no matter how 
large or small, is divided into 360°, each degree into 60', 
and each minute into 60" (seconds), or the whole circle 
into 1,296,000". Now, if we take a circle of the dia¬ 
meter of one inch, or say one foot, we have to employ the 
microscope to detect the dimension of the visual angle 
of one second; but if we take a circle with a radius of 
the moon’s distance from the earth, then each second 
will represent one mile. We must bear in mind that 
this mathematical division of a circle in degrees, minutes 
and seconds is not an exact measure in inches, feet or 
miles, but only indicates the proportional part of any 
circle, be it small or large. It is very important to re¬ 
member this, as it facilitates the calculation regarding 
the sizes of each test-letter for the different distances. 
Let us take for instance, the letter of C C. It should be 
seen at 200 Parisian feet from us, which is the radius of 
a circle, whose diameter is 400 ft. in length, and to find 
the circumference of the circle we have to multiply 400 
by 3.14, which equals 1256 feet. This is the length of 
the circumference of that imaginary circle drawn around 
us 200 feet from our eye, as the center of this circle. 
We have to divide these 1256 feet by 360, to find the 
length of one degree, which = 3.49 ft. or 41.88 inches, 
and to find the length of one minute, we divide them 
again by 60, which =' 0.698 inches. This is the width 
of each of those twenty-five little squares we see faintly 
indicated by dotted lines beneath that test-letter. 

Snellen selected the Koman block-letters because every 
line is of an equal thickness. Some of them are espe¬ 
cially adapted for a test; for instance, to distinguish 


92 


HAND-BOOK FOR OPTICIANS. 


O from C or G, and B from R or E, or P from F, we must 
be able to see plainly the space of one of these little 
squares, as it is the characteristic distinction of one letter 
from the other. According to the different distances we 
occupy before the test-types, these squares alter in size, 
and limit also the thickness and height of the letters, as 
each stroke of them is of the width of those squares. 
Snellen found by numerous experiments that a normal 
eye just could detect one of them at the given distance, 
but in order to relieve the eye from all strain, and enable 
it to see the object distinctly for a length of time, he 
enlarged the letter in each direction five times, and only 
indicated beneath it the size of one minute by those 
dotted lines. The letters themselves represent, there¬ 
fore, a visual angle of five minutes. 

We have seen before, that one minute of the first test- 
letter was equal to 0.698 inches, therefore, five minutes 
will be = 3.49" in Parisian measure. To reduce the 
Paris inches into American, we multiply them by 39.37 
and divide by 37. The following table is calculated this 
way, and shows the true American measure of every 
letter at the different distances. Although Snellen confined 
his test-types to the following distances: 200,100, 70, 50, 
40, 30, 20, 15 ft., etc.; several parties have reproduced 
them by adding intermediate sizes, which I include in this 
list to enable opticians, who make use of such types, to 


measure them and see if they are correct. 

200' CC = 3.49 Paris or 3f American inches. 

160' CLX = 2.80 “ 3 

120' CXX — 2.09 “ 2J 

100' C == 1.74 “ 1! 

80' LXXX = 1.40 “ H 

70' LXX = 1.22 “ li 

60' LX = 1.04 “14 

50' L = .87 “ U 

40' XL « .70 “ S 

30' XXX == .52 “ 

20' XX .35 “ f 

15' XY = .26 ‘‘ -h 

10' X == .17 “ * 


USE OF TEST-TYPES. 


93 


The same way we have to reduce the distance into 
American feet; for instance, 20 Paris feet are 21.3 
American. Snellen’s suggestion, that the rays from a 
distance of twenty feet could be considered parallel, can 
only be admitted from a practical view, and is the utmost 
limit in this regard. Any shorter distance will tax the 
accommodation and will not give a satisfactory result. 

In using the test-types we should have a room fully 
twenty feet long from the door or window to the opposite 
wall, where we should fasten the types about four or five 
feet from the floor. The room must be, in good weather, 
well illuminated and the types clearly seen. We place 
our customer 20 or 21 feet before the types. If he sees 
No. XX, his visual power (V) is normal and is expressed 
by the formula Y = 20/XX; but if he sees only XXX, 
then his vision = 20/XXX. The numerator is always 
the distance, and the denominator is the type he can 
read. If he only reads CC, then Y = 20/CC. Some 
eyes may be able to see XY, or even X at twenty feet, 
then their formula is Y = 20/XV or 20/X. We cannot 
make a mistake in the marking of the visual power, 
when we impress our memory with the general formula, 
Y = d/t; d stands for distance, and t for type. 

To attain a more exact formula, we can make use of 
the si<ps + and —. If, for instance, a patient can see 
No. XXX, and one or two letters of XX, his formula 
can be expressed by 20/XXX -f~, but if he misses one 
letter of XXX, seeing perfectly XL, then his formula 
V = 20/XXX —. Such formulae, when carefully 
recorded, can be utilized two-fold: they serve as refer¬ 
ence for future measurements of the eye, and also as a 
guide for the selection of suitable glasses. As to the 
latter purpose we only invert the formula, and substitute 
“diopter” for “type,” viz: 

20/XXX or § - | i +. 1.50 s 

, 20/XL i4 -j- -4- y = -f- 2.5 

20/L “ f -s- | = + 2.505 

20/LXX “ \ -f- i *?;■+. 3.505 

20/C + 5.5 

20/CC “ = + 10.5 


94 


HAND-BOOK FOR OPTICIANS. 


The employment of the signs -f- and — will necessitate 
the trial of the intermediate numbers of lenses not men¬ 
tioned here. 

Although Snellen’s test-types include the finest letters 
to test the eyes for near vision, they do not answer this 
purpose as well as Jaeger’s, which should be used always 
in connection with them. Yet, with both test-types an 
absolute accuracy is not attainable; we must be content 
with the average statement that a person who still reads 
No. 1 Jaeger at ten inches from the eye, and No. XX of 
Snellen’s test-types at twenty feet, has normal vision 
and is not in need of spectacles. 

Let me mention here the method which we old opticians 
formerly made use of in calculating the strength of 
glasses for presbyopia as well as for myopia. We 
made the patient read ordinary print, and then measured 
in inches the distance from the eye to the paper. The 
optician was standing near the customer with a rule in 
his hand to measure the length of his reading distance. 
For those parties who could not read we made use of 
Lehot’s device, consisting of a simple black rule, three 
feet long, with a white thread strung over its middle from 
end to end. ,We placed one end of this rule horizontally 
upon the chin of our customer, and directed him to slide 
his finger along the rule to that point where he could see 
the thread most distinctly. There, apparently, was his 
focus, and there he saw the thread single, while on this 
side and beyond the finger it appeared double. This 
experiment is based upon the same phenomenon as that 
of the finger and the pencil; if we hold vertically a 
finger fourteen inches before our eyes, and a pencil at 
seven inches, then, in looking at the finger, we will see 
two pencils, but looking at the pencil we see two fingers. 

Having thus ascertained the length of his focal dis¬ 
tance, we made use of the old rule that all eyes with a 
longer focus than ten inches, had to use convex glasses, 
and those with a shorter focus, concave glasses. To find 
now the proper lens, we multiplied the length of distinct 
far vision by ten as the standard near-point, and divided 
the product by the difference of the far and near points. 
For instance, if somebody could 'see best at 14", we 


USE OF TEST-TYPES. 


95 


multiplied fourteen by ten, and divided the product by 
the difference of the two numbers by four: 

14X10 _ 140 | qc • , 

“14 _ jo — —= H- oh inches; or 

24X 10 240 , _ . , 

24 — tin ~ " 14 “ — • |- 1 ( inches. 


When the focal distance was shorter than ten inches, 
say eight or six, we made our calculation in this way: 


1 0 X 8 
10 — 8 
10 X o 
10 — 6 


— 40 inches; or 

— 15 inches. 


The strength of those spectacles were generally near 
enough to commence with as a trial; but as they rep¬ 
resented only the average sight of both eyes combined, 
any regular optician wisely made the necessary allowance 
for a casual difference in the eyes. — It amuses me now 
to look back on something I once considered to be strictly 
scientific, and which is at present thrown aside as unre¬ 
liable and obsolete. 






CHAPTER XIII. 


Refraction and Dispersion of Light. 


The ancients supposed light to be produced and vision 
excited, by something emitted from the eye. The mod¬ 
erns hold vision to be excited by something that strikes 
the eye from without. Newton supposed light to consist 
of small particles shot out with inconceivable rapidity by 
luminous bodies, and fine enough to pass through the 
pores of transparent media. Crossing the humors of the 
eye, and striking the optic nerve, these particles were 
supposed to excite vision. This was called the emission 
theory of light, and found many strong supporters, among 
others, Laplace, Malus and Brewster. This theory was 
first opposed by the astronomer Huyghens, and after¬ 
wards by the celebrated mathematician Euler; but it 
maintained its ground until it was finally overthrown by 
the labors of Thomas Young and of Augustin Fresnel. 
These two eminent philosophers separately succeeded in 
establishing the wave or undulatory theory of light, by 
which all optical phenomena can be explained. They 
compared light with sound, the main difference being 
their relative velocity of propagation. The waves of 
sound require an elastic, dense body, like our atmosphere 
to make an impression upon the ear, so differing from 
light, which transmits its waves by a substance of extreme 
tenuity, called ether. 

Ether is merely the name of something we know not 
what, but we know that without its presence, we have to 
drop the wave-theory. Newton’s theory did not require 
such a vehicle of light because the velocity of his light- 
missiles were not obstructed by a vacuum, but rather ac¬ 
celerated. It was formerly supposed that the space be¬ 
tween the stars was perfectly vacant, judging from our 
own atmosphere, of which the last trace disappears at a 




REFRACTION AND DISPERSION OF LIGHT. 


97 


height of about two hundred miles from the earth. The 
promotion of the wave-theory compelled its supporters 
at once to fill the universe with some medium to carry 
the waves of lisrht and continue their motion. 

The word undulation is from the Latin, unda , a wave, 
and undxda , a little wave. The selection of this word is 
not a good one, as it leads the student to confound the 
vibrations of light with the up and down motion of agi¬ 
tated water, without indicating how one molecule imparts 
a forward movement to other molecules. I would pro¬ 
pose to call it the vibratory theory , and would explain its 
propagation by the presence of heat, as light and heat 
are inseparable. Light is a high potency of heat, and as 
heat expands everything, the molecules of ether sur¬ 
rounding and penetrating the source of light ought to be 
expanded. But as the theory of the nature of ether ex¬ 
cludes the presence of pores between its molecules, there 
is no room for such an expansion; therefore, in its fran¬ 
tic effort to obey an omnipotent law of nature, it only 
can push the next molecule, or rather a countless line of 
them, according to the power of the first impulse it re¬ 
ceives. Its apparent expansion and alternate contraction 
without changing its place, is called vibration , and is of 
such a rapid succession that we can form no true concep¬ 
tion of it; we have to express the vibrations by billions 
each second. The extreme tenuity of the ether facili¬ 
tates the rapid propagation of light, and the continuous 
impulses, its steady advance (192,000 miles a second). 

We must beware of the wrong idea, that the atoms of 
ether are flying about, which is an impossibility, as the 
whole Universe is equally filled with it. It does not im¬ 
pede the progress of any moving body, but light sets it 
to an oscillating, and instead of imparting its impulses 
pell-mell to the surrounding molecules, it always takes 
the shortest line, the straight one. 

When we speak of ether, we imagine it to be the fin¬ 
est matter in existence, whose molecules are indivisible; 
itself being without motion, allows the world to move in 
it without the least impediment. And still there must be 
something finer to fill the space between its atoms, be¬ 
cause all molecules are considered to be of a rounded or 


98 


HAND-BOOK FOE OPTICIANS. 


spherical shape, leaving yet room for something finer. 
The subtile argument that there is no limit to the divisi¬ 
bility of matter has little weight, as it would totally de¬ 
stroy matter for the sake of a 4 ‘theory.’’ 

Light is interrupted in its direction only by entering a 
transparent medium of different density from that 
through which it previously moved. This change of the 
rays of light from their direct course is called the re- 
fraction of light. John Tyndall explains this phenome¬ 
non by the following illustration: “Suppose light to im¬ 
pinge from air upon a plate of glass, the wave will be re¬ 
tarded on passing into a denser medium. If this wave is 
oblique to the surface of the glass, that end of the wave 
which first reaches the glass will be first retarded, the 
other portions being held back in succession. This re¬ 
tardation of one end of the wave causes it to swing 
round, so that when the wave has fully entered the glass 
its course is oblique to its first direction; it is refracted . 
If the glass into which the wave enters be a plate with 
parallel surfaces, the portion of the wave which reached 
the upper surface first, and was first retarded, will also 
reach its under surface first, and escape earliest from the 
retarding medium. This produces a second swinging 
round of the wave, by which its original direction is res¬ 
tored. In this simple way the wave-theory accounts 
for refraction.” 

The following illustration will explain it more practi¬ 
cally. 



The figure abed represents a plate of glass with 








REFRACTION AND DISPERSION OF LIGHT. 


99 


parallel surfaces; l is the incident ray which enters the 
plate at i, where we erect the perpendicular q. At 
the height above the slab equal to its thickness, we draw 
the line op', at right angle to p q, thus forming the rect¬ 
angular triangle o ip. The angle at i is called the angle 
of incidence, and o p is called the sine of this angle. 
After the ray enters the plate, it is bent towards the per¬ 
pendicular till it reaches e, forming another triangle er q . 
The angle at r is the angle of 7'efraction, and e q the sine 
of this angle. 

We see here plainly that the angle of incidence is larger 
than the angle of refraction; in glass, for instance, in 
the proportion of three to two, as seen in the illustration. 

The relation of the angle of refraction to the angle of 
incidence, though the same for each substance, varies with 
the nature of different media, each of which has a distinct 
power. The ratio or proportion between them is called 
the index of ref raction. For different media, it is as 


follows: 

Air.1.000 

Water.1.33(1 

Oil of Turpentine.1.475 

Crown Glass.1.538 

Rock Crystal.1.548 

Flint Glass.1.633 

Strass or Paste.2.028 

Diamond.2.439 


In this table, air is taken as the unit of comparison. 
The refractive power of crown glass and pebbles is almost 
the same; flint glass shows a considerable increase, strass 
even more so. The latter is also a flint glass with a 
larger proportion of lead, and is known as the extra white. 
The high refractive power in diamonds causes that spark¬ 
ling clearness called ‘ ‘first water’ ’, and is much appreciated 
by all connoisseurs of precious stones. Spectacle lenses 
made of diamond would be injurious to the eyes on ac¬ 
count of this glaring refractive power. 

The refraction of the rays of light passing from one 
medium to another also causes the separation of light into 
its different colored rays. This is called the dispersion 
of light. We have seen that refraction refers to the 










100 


HAND-BOOK FOR OPTICIANS. 


change in the direction of the rays, while dispersion 
relates only to colors, produced by an unequal bending of 
the rays of light. This is best shown by means of a 
prism. The waves of ether generated by luminous bodies 
are not all of the same length; some are longer than 
others. In refracting substances the short waves are 
more retarded than the longer ones; hence, the short waves 
are more refracted than the long ones. The luminous 
image formed, when a beam of white light is thus decom¬ 
posed by a prism, is called a spectrum. If the light em¬ 
ployed be that of the sun, the image is called the solar 
spectrum. 

The color of light is determined solely by its wave-* 
length; color, is to light what pitch is to sound. The 
pitch of a note depends on the number of aerial waves 
which strike the ear in a second; the color of light de¬ 
pends on the number of ethereal waves which strike the 
eye in a second. Thus the sensation of red is produced 
by imparting to the optic nerve a certain number of im¬ 
pulses per second; while the sensation of violet is pro¬ 
duced by imparting to the nerve almost twice as many 
impulses in the same time. The waves of the extreme 
violet are about half the length of those of the extreme 
red, and they strike the retina with double the rapidity 
of the red. While, therefore, the musical scale , or the 
range of the ear, is known to embrace nearly eleven oc¬ 
taves, the optical scale , or range of the eye, is comprised 
within a single octave. 

The dispersive power varies in different bodies; it is in 


Rock Crystal.0.026 

Water.,. .0.035 

Crown Glass.0.037 

Oil of Turpentine.0.042 

Flint Glass.0.049 

Diamond.0.056 


This table of the index of dispersion shows clearly the 
superiority of pebbles for spectacles over any glass, be¬ 
cause the eye is most benefited in the length of time by 
lenses of the lowest power of dispersion. The optical 
glass of telescopes and microscopes is an exception to this 
rule; such glass must be of the possible highest refract- 








REFRACTION AND DISPERSION OF LIGHT. 


101 


ive power, but its dispersion is neutralized by a certain 
combination of lenses. The limited use of scientific in¬ 
struments and their special object, constitute the princi¬ 
pal difference between them and the constantly employed 
spectacles. 

If spectacles could be set with achromatic lenses, like 
objectives of spy glasses, they would be still better than 
crown glass for the eye, but nobody will carry such a 
weight on his nose; besides, the high price of such lenses 
would permit only a limited sale, and therefore, no opti¬ 
cian could keep an assortment of them in stock. 

The high dispersive power of diamonds causes the fas¬ 
cinating display of beautiful spectral colors, called “first 
fire,” which combined with the “first water” makes this 
mineral the “King of Gems.” 


CHAPTER XIV. 


Achromatic Lenses. 


Opticians and dealers in optical instruments are often 
asked by inquisitive customers to explain the difference 
between an achromatic and non-aehromatic instrument. 
I, therefore, think it proper to devote a chapter on 
Achromatism, not only to enable my readers to give 
satisfactory answers to all questions regarding this sub¬ 
ject, but also to make them better judges in selecting 
their instruments for the stock in trade. — Nobody will 
expect here a scientific treatise on this subject, as it 
would involve some of the highest mathematical prob¬ 
lems and calculations when applied to scientific and 
astronomical instruments. I confine myself merely to 
the primary elements of this interesting study, to keep 
within the limits of a Hand-Book for workmen. 

Achromatism is derived from the Greek word chroma , 
meaning color; therefore, chromatic , full of color, and 
achromatic , free from color. Before the invention of 
achromatic lenses, the astronomers were much annoyed 
by the colored borders their instruments showed around 
the objects, which made them appear indistinct and 
blurred. The stronger the lenses, the more visible was 
this so-called chromatic aberration. Take, for instance, 
a convex lens of four inch focus and a sheet of white 
paper; let the direct, or reflected sunlight fall on the 
lens, and observe the luminous circle on the paper before 
the lens is in its focus. We see there a distinct blue 
border in the circle; but when we approach the lens 
towards the paper, the luminous circle gets Gradually 
smaller till the lens is in its focal point; then°the blue 
color disappears. When we continue to approach the 
lens towards the paper, the focal point enlarges ao*ain, 
but now the inside border is red. To understand this 




ACHROMATIC LENSES. 


103 


phenomenon we must remember that white light consists 
of seven differently colored rays, having different degrees 
of refrangibility. The violet and blue rays unite first 
into a focus, then the green, yellow and at last the red, 
and only at the middle of these different foci (at green), 
by the mixture of all the colors, they appear to unite 
into a common focus apparently without color. Before 
we reach this principal focus of the lens, the red ray 
produces the luminous disk, and when we reach the 
special focus of yellow, the blue ray makes its appear¬ 
ance at the border of the disk, anxious to join the former 
at the focus of green, which is the only colorless point 
of the spectrum. But when we reach the shorter focus 
of the blue ray, which now produces the luminous disk, 
then the red ray, as the predominant color of the spec¬ 
trum, makes its appearance on the inside border of the 
disk. This lack of power on the part of a convex lens 
to bring the differently colored constituents of light to a 
common focus, is called the chromatic aberration of the 
lens. 

A weaker lens, say of thirty-inch focus, does not de¬ 
monstrate this phenomenon as well as a stronger one. 
This is the reason, why telescopes, before the invention 
of achromatic lenses, were of such an enormous size, 
sometimes of several hundred feet in length. The chro¬ 
matic aberration is also seen in cheap opera and spy¬ 
glasses with single ocular and objective lenses. 

The ingenious Newton by his experiment of interposing 
a prism in the way of the solar beam, admitted through a 
small hole into a darkened chamber, made it produce on 
the wall, not a white circle, as it would have done if 
allowed to pass on without interruption, but an elongated 
image, or spectrum, as he called it, displaying the rain¬ 
bow colors. This phenomenon proved the hitherto un¬ 
suspected facts, first, that white or common light is, in 
reality, composed of seven different species of rays; and 
secondly, that each of these rays is refrangible in a 
different degree from the other on passing into a new 
medium, taking a separate course of its own, so that the 
beam spreads out into the resemblance of a fan. This 
is called the divergence, or dispey'sion of the rays of light; 


104 


HAND-BOOK FOR OPTICIANS. 


and, from some other experiments which he made, he 
was induced tp believe that whatever transparent sub¬ 
stances, or media, refracted a beam of light in the same 
degree, or changed in the same degree its general direc¬ 
tion, were also equal in their dispersive powers, or 
made the different rays separate from one another to 
the same extent. From this followed a very important 
consequence. The magnifying powers of the common 
telescope depended entirely upon the refraction of the 
light in its passage through the several lenses; but it 
could not undergo this operation without the rays being 
at the same time dispersed; and this necessarily threw a 
certain indistinctness over the image which such tel¬ 
escopes presented to the eye. Here, therefore, was 
apparently a defect in the refracting telescope which 
admitted of no cure; for the dispersion bearing the 
same relation in all substances to the refractive power, 
we can not obtain the requisite refraction without its 
inseparable companion, the same amount of dispersion. 
It was this consideration which made Newton give up all 
thoughts of improving the refracting telescope, and apply 
himself, as Gregory had done, to the construction of one 
which should present its image, not by refracting, but 
by reflecting the light from the object. He, therefore, 
constructed a mirror-telescope, a “reflector,” of the 
magnifying power of forty diameters, which he after¬ 
wards presented to the Royal Society of London. By 
using a mirror, instead of refracting lenses, he overcame 
the annoying chromatic aberration to a great extent. 

The renowned Euler, on the contrary, propounded 
(1747) the idea of the possibility to overcome chromatic 
aberration by a combination of spherical lenses of differ¬ 
ent density; and the Swedish mathematician, Klingen- 
stierna, demonstrated this idea scientifically, so thalTten 
years later the English optician, John Dollond, 
manufactured the first achromatic spy-glass.* He made 

* Another Englishman, Chester M. Hall, is also credited with having 
made achromatic lenses as early as 1729, but his invention was not noticed. 
He was a wealthy man and seems to have been careless of fame; at least 
he took no trouble to communicate his invention to the world. But after 
the patent rights were granted to Dollond, other instrument-makers 
disputed his original claim to the invention, and it was left to the court 
for decision. Lord Mansfield, who tried the case, ruled that “it was not 
the person, who locked his invention in his scrutoire that ought to profit 
kin? ” UCh mventlon ’ but he who brought it forth for the benefit of man- 



ACHROMATIC LENSES. 


105 


use of Euler’s suggestion, that the human eye was 
achromatic on account of the different densities of the 
crystalline lens and the vitreous humor. He selected two 
kinds of glasses which represent similar differences in 
density, viz.: -flint and crown glass, and ground the denser 
flint glass into a weak concave lens, and the lighter 
crown glass into a strong convex lens, which thus com¬ 
bined produce a colorless focus. — When we take the 
objective lens of an opera glass, and make the same 
experiment as we made before with the single spherical 
lens, we will not see the red or blue border at the differ¬ 
ent distances of the lens from the paper, but will observe 
only a white disk or circle without color. — The experi¬ 
ment with the single spherical lens also proves that our eye 
is not perfectly achromatic, as was believed formerly by 
many scientists, also by Euler. The eye suffers from 
chromatic aberration as well as from spherical aberration; 
the latter relates to the imperfect focusing of the rays 
falling on a spherical lens. The rays nearest to the center 
of the lens produce the principal focus, which is always 
longer than the foci of those rays passing through the 
lens near its periphery. The effects of this spherical 
aberration is obviated in an instrument by means of a 
diaphragm , which is a blackened shield with a small 
opening in the middle, permitting only the center-rays 
to pass, thereby shutting off the peripheral rays, and 
preventing spherical aberration ; but -the chromatic 
aberration in instruments can be obviated only by 
achromatic lenses. 

Our eye has similar contrivances to guard to a certain 
extent against these imperfections; the iris performs the 
duty of the diaphragm, and the different densities of the 
refracting media produce the achromatism. We can 
greatly diminish the spherical aberration of our eye by 
looking through a hole in a card made by a pin, but we 
cannot altogether remove its chromatic aberration. The 
best experiment to show the comparatively high degree 
of chromatic aberration in our eye is that with a deep 
blue glass. If we look at the flame of a candle through 
such a glass, the flame looks bluish-violet at the length 
of distinct vision (focus). When we approach towards 


106 


HAND-BOOK FOR OPTICIANS. 


the light, the color of the flame remains violet, but has 
now a red border. When we withdraw beyond distinct 
vision, the violet flame turns red, surrounded by a blue 
halo which broadens the more we remove from the light. 
I believe, my readers will understand this phenomenon 
without further explanation, as it is analogous to the first 
experiment we made with a single spherical lens in sun¬ 
light. 

Now, let us return to the inventor of the achromatic 
lenses, to John Dollond. We must not imagine that his 
accomplishment was an easy task, it required many 
tedious experiments and trials, before he found the pro¬ 
portionate strength of each lens, because the greater 
dispersive power of flint glass had to be counter-balanced 
by a less curvature, and the less dispersive power of 
crown glass by a stronger curvature of the lens; in other 
words, the index of refraction has to be in proportion to 
the index of dispersion. The following combination 
illustrates the above rule: a crown glass of -f-2§ inch 
focus, and a flint glass of — 4J inch focus will give an 
achromatic lens of + 6-^j- inches; the crown glass must 
be always the stronger lens. These relative proportions 
were later on carefully calculated and tabulated by 
Herschel, Fraunhofer, Littrow, etc., but Dollond, at his 
time, had no tables to go by, he had to experiment and 
try till he succeeded. 

His son, Peter Dollond, and the English optician, 
Ramsden, made great improvements in this direction, 
especially in the manufacture of achromatic microscopes 
and astronomical telescopes. 

Fraunhofer revolutionized the old methods by the in¬ 
vention of his clearer flint glass in large pieces, which en¬ 
abled him to shorten considerably the inconvenient length 
of telescopes, and by using larger objective lenses he pro¬ 
duced instruments of great power and perfection. At 
the suggestion of the astronomer Littrow, another im¬ 
provement in the construction of telescopes was made by 
the optician Ploessl at Vienna; he did not join the flint 
and crown glass together as we find them still in opera 
glasses, but distanced them in a certain proportion, by 
which method it was again possible to shorten the tubes 


ACHROMATIC LENSES. 


107 


of telescopes. He called his instruments “ dialytic tel¬ 
escopes,” or simply Dialytes . 

Among the most noted manufacturers of fine achro¬ 
matic instruments are Yoigtlander at Vienna, Lerebours 
& Secretan (now Eichens) at Paris, A. Boss (now Dall- 
meyer), and Beck Bros, at London; also the brothers 
George and Adolf Repsold, at Hamburg, who mounted 
the great Russian telescope at Pulkowa, a refractor of 
thirty-inch aperture, whose objective lens was ground by 
Alvan Clark & Sons. — The greatest achievement in the 
line of astronomical telescopes is that at the Lick Observ- 
atory in California, a triumph of American workmanship. 
The names of Clark & Sons, and of Warner & Swasey, 
who mounted the instrument, will always be remembered 
as prominent American opticians and mechanics. In 
fact, the U. S. do not come short in the general race 
for the superiority in the manufacture of optical instru¬ 
ments. There is David Rittenhouse of Philadelphia; 

• “ Chas. A. Spencer of Canastota of N. Y.,” famous for 

the excellence of his microscopic objectives; Henry Fitz 
of New York, a skillful telescope-maker; W. Wales at 
Fort Lee, N. J., J. & W. Grunow of N. Y., James W. 
Queen and Jos. Zentmayer, both'of Philadelphia; Bausch 
& Lomb of Rochester; all of them can be counted master 
opticians. But the most skillful optician in the world 
was without doubt the late B. B. Tolies of Boston; his 
achromatic objectives are the finest ever produced,’and 
command the highest prices. 

Only since the invention of achromatic lenses we are 
able to manufacture the various instruments for scientific 
investigations; but in order to produce a glass suitable 
foi instruments of ^ieat accuracy, it was necessary that 
the glass-industry closely allied itself with the queen of 
all sciences, “mathematics,” by which it was itself 
elevated from the former lowly position to that of a 
queenly art. Step by step, we thus can refract or dis¬ 
perse light at our will; we can produce either a blazino- 
fire or a beautiful picture; we can explore the minutest 
organic or inorganic substance, and solve the mysteries 
of endless space. By means of glasses we can analyze 
the constituent parts of the sun and other celestial bodies, 
and record their ever changing phenomena. 


s 


CHAPTER XY. 


i 

Anatomy of the Human Eye. 


Inventions have occasionally explained the workings of 
the organs of our body; for instance, the bellows demon¬ 
strate the action of the luogs, the pump that of the 
heart, the camera obscura that of the eye, etc. Men 
used their eyes for many thousand years without the 
slightest idea of their real mechanism, until the invention 
of the camera, by an Italian, Battista Porta, about three 
hundred years ago, gave them a fair explanation of the 
workings of this organ. Since then it gradually dawned 
on the minds of scientists, that this implement explained 
the mechanical workings of our eye better than any 
theory heretofore promulgated. Porta himself compared 
his instrument with the eye, but falsely attributed to the 
crystalline lens the duty which is performed by the 
retina. About the year 1611, the German astronomer, 
John Kepler, explained the real relationship of the lens 
to the retina, and gave a satisfactory explanation of the 
action of convex and concave glasses. Notwithstanding 
the most elaborate researches of later scientists and espe¬ 
cially of anatomists, to interpret the entire process of the 
act of seeing, they only succeeded to trace the connection 
of the eye with the cerebrum, and even as far as to the 
cerebellum, but the mysterious part which the brain has 
to perform is yet a sealed book for many centuries to 
come, perhaps forever. — 

The two large sockets or orbits of the eyes as seen in a 
skull are filled in a living being with muscles, small 
bloodvessels and cushions of fat, leaving only room for 
the eyeball and the lids. The direction of the axis of 
the eyeball does not correspond with that of the orbit; 
the axes of the eyes are parallel with one another, while 
those of the orbit diverge considerably in front, and if 



ANATOMY OF THE HUMAN EYE. 109 

prolonged backwards, would intersect at an acute angle 
a little distance before the middle of the forehead and 
occiput. Hence, as the optic nerves coincide in their 
direction with that of the axes of the orbits, each of them 
enters the globe of the corresponding eye to the inner 
side of its axis, and, consequently, of the axis of vision. 
The globe of a normal eye is perfectly round, with the 
exception of the cornea , which forms a slight elevation. 
When we shut the eye, and press one finger gently on 
the upper lid, we can plainly feel this elevation by mov¬ 
ing the eye. The cornea extents to the outside border 
of the iris, and is easily seen by looking at the eye of 
somebody obliquely or in profile, when the iris appears 
as a straight vertical line, and the cornea as an elevated 
but transparent section of a smaller ball laid upon a 
larger one. The iris, which presents the colored circle 
seen through the transparent cornea, resembles a partition 
placed vertically so as to divide, but very unequally, the 
interval between the cornea and the lens into two parts. 
This interval is filled by the aqueous humor , which en¬ 
ables the iris to move freely in any direction. The 
space between it and the cornea is called the anterior 
chamber, that behind it is the posterior chamber; the first 
is the largest, and both communicate through the pupil. 
The iris has different functions to perform; it acts like 
a diaphragm in a spy-glass, as a screen or curtain, to 
prevent light from falling on the outer part of the 
crystalline lens, which otherwise would cause an annoy¬ 
ing spherical aberration. It also regulates the quantity 
of light entering the eye by closing or opening the pupil, 
which is done by the contraction of its different fibres, 
either of the circular or radiating ones. When the 
circular fibres contract, the pupil gets smaller, the con¬ 
traction of the radiating opens the pupil. The pupil is 
only a round opening in the center of the iris through 
which the light enters the eye. The action of the iris is 
not controlled by our will, but is regulated by the sym¬ 
pathetic nerves. The black color of the pupil is partly 
due to the pigment covering the whole inside of the eye¬ 
ball, except the retina and lens.* Eyes without this pig- 

* See Chapter XVIH, “The Ophthalmoscope.” 



110 


HAND-BOOK FOR OPTICIANS. 


ment have a red pupil from the reflection of the choroid,* 
as we see in rabbits and some birds, also in albinos. The 
retina , as the optic nerve is called after having entered 
the eyeball, is spread over the back portion of the eye, 
receiving the impression of light and conveying it to the 
brain of which it is merely a projection and is, therefore, 
in direct communication with it.* Each eye has its 
separate nerve, but before the two optic nerves enter the 
brain they apparently cross each other, which explains 
why one diseased eye very often affects the other; it 
also accounts for the great sympathy of one eye with 
the other. The optic nerve of each eye, separately, 
runs upward towards the base of the skull, and passes 
through one of the two small openings of this bone into 
the brain chamber, but they meet here before they are 
absorbed by the brain. This connecting point is called 
the “commissure.” Some fibres of the nerves do not 
proceed any further, but are turned to the other eye, 
i. e ., a few fibres from the nerve of the right eye branch 
off and go to the left eye, and vice versa. At the com¬ 
missure there is still another interchange of the fibres^ 
the larger part of the optic nerve of one eye carries along 
with it a smaller portion of the fibres of the opposite 
nerve. After their entrance into the brain, they can be 
followed up to some extent, but as the fibres are freely 
dispatched, right and left, to different parts of the brain, 
the nerves are quickly reduce^ in size, visible only by a 
microscope, and the anatomist loses at last all traces of 
them. The seat of sight is not yet detected. 

* Starting from the junction of the retina with the vitreous humor, we 
have : 

1. The layer of nerve fibres; 

2. the layer of nerve cells; 

3. the granular layer; 

4. the inner granular layer; 

5. the intermediate layer; 

6. the outer granular layer; 

7. a second fine membrane; 

8. the layer of rods and cones; 

9. the black pigment of the choroid, which communicates with ;tlie 
sclerotica. 

The light, therefore, has to penetrate seven different layers before it 
sets in motion the rods and cones, which are considered to be the most 
active parts of the retina. For many years I was in error as to their true 
location, misguided by incorrect illustrations. 



ANATOMY OF THE HUMAN EYE. 


Ill 


To come to a thorough understanding of the anatomy 
of the eye, it is necessary to dissect one of a slaughtered 
animal. The nearest to the human eye is that of a hog 
or calf, in size as well as in the whole arrangement of its 
component parts, and which we can readily procure from 
our butcher. In order to facilitate such a dissection, we 
ought to have a pair of tine pointed scissors, a sharp 
knife (a razor is very handy), a pair of pointed and also 
flat forceps or pincers, and a few common pins, bent 
into hooks, with a thread attached to be fastened around 
some nails which we have hammered half their length 
into the so-called “ dissecting board.” After having 
received the eyes, we lay them upon the board and 
examine first the outside of the eyeball. In case the 
butcher was well instructed and has left a piece of the 
optic nerve on it, we will see that the eyeball resembles 
an apple with its stem, which is not exactly opposite the 
pupil, but is attached a little to one side. We find also 
the remnants of the six muscles which moved the eye, 
and which had to be cut before extracting the ball from 
its socket. They are attached to the sclerotica about the 
middle from the pupil to the optic nerve. Next, towards 
the front part of the eye, we find a loose membrane 
severed from the inside of the eyelids with which it was 
connected, called the conjunctiva. This membrane is a 
fine transparent skin covering that part of the eye we see 
in a living being, and which forms also the inside lining 
of the eyelids, thus protecting the backpart of the eye¬ 
ball from entering of any fluids or solid objects. We 
detach this membrane from the ball, which is easily done 
as it adheres only loosely to it. After the removal of 
the conjunctiva we direct our attention to the cornea , a 
transparent lamina or scale, the curved covering over 
the pupil and iris, and the only transparent part of the 
sclerotica, having the shape of an old fashioned watch 
glass. With a sharp-pointed knife, or with the razor w,e 
can cut a part of it away without touching the iris. One 
or two drops of water, which filled the space between 
the cornea and lens, will be spilled, and in examining now 
the pupil, we find it to be only an opening in the iris. 

Before we go on in this direction and explore the iris 


112 


HAND-BOOK FOR OPTICIANS. 


and crystalline lens, we will turn our attention to the dif¬ 
ferent coatings of the eyeball, and thus enter its interior 
sideways. The sclerotica covers the whole eye, and is 
with the exception of the cornea opaque. It is a tough, 
leathery membrane of a bluish or yellowish white color, 
capable of enduring many injuries without breaking. We 
will find it a troublesome job to remove a piece of it 
without injuring the next layer, the choroid , which is a 
rather tender membrane as it consists almost entirely of 
small blood vessels, covered with the so-called pigment. 

Any injury from outside, for instance a heavy blow, 
may rupture some of its arteries, and give the white scle¬ 
rotica a reddish or bloodshot appearance. In removing 
the choroid we tear likewise the pigment, which loosely 
covers the choroid, and yields readily to the scraping of 
a blunt instrument or the fingernail. It has the appear¬ 
ance of a mixture of potblack and lard. Particles of this 
pigment sometimes lose their hold upon the choroid, and 
float in the vitreous humor, causing the annoying sensa¬ 
tion of seeing flies (muscse volitantes) apparently be¬ 
fore the eye, which are generally of no importance, as 
every eye is more or less subject to this occurrence, and 
in most cases vision is not affected by them. The next 
part of the eye we come to is the vitreous humor, occu¬ 
pying three-fourths of the interior of the ball. It will 
ooze from the lateral opening we have made, and lie upon 
the dissecting board as a quivering, perfectly transparent 
mass, like jelly. We now cut the empty shell into two 
halves, front and back, and examine the back one. We 
see here the retina expanded in a circular, spherical 
form, slightly indented where it enters the eye. We 
should think this spot to be the most sensitive to light, 
but strange to say, we cannot see there at all, it is the so- 
called blind spot of the retina. The most sensitive por¬ 
tion of the retina is a small space, a little outside of the 
blind spot, and exactly in the line of direct vision, called 
the yellow spot (macula lutea). 

There is now left the upper half of the eyeball for our 
examination. After having it dipped in water we hold 
it over some print, when we observe that every letter is 
magnified. This is due to the crystalline lens, a structure 


ANATOMY OF THE HUMAN EYE. 


113 


more consistent than the vitreous humor, and is sur¬ 
rounded by a transparent membrane, the capsule. The 
lens extends below the iris; this is a projection of the 
choroid, closely connected with the ciliary muscles , and 
is also called the ciliary processes. Any contraction or 
relaxation of these muscles changes the size and position 
of the lens, causing the so-called “accommodation” of 
the eye. We understand by this the faculty of the lens 
of adjusting its focal distance for near and far objects, 
producing the same effect as when we lengthen or shorten 
an opera glass by means of the screw. The only dif¬ 
ference is that the eye adjusts itself without such an ap¬ 
pliance, as the ciliary muscles attend to this changing. 
We now make a slight cut over the whole length of the 
lens, carefully severing only the inner side of the cap¬ 
sule, and by a gentle pressure cause the lens to jump out, 
which will keep its original shape, that of a strong con¬ 
vex lens, but will yield readily to the pressure of our 
finger. It consists of fine layers of minute tissues and 
contains comparatively but little fluid. In case of cata¬ 
ract it solidifies, and after its extraction from the eye 
crumbles between the fingers like dry cheese. We now 
remove the empty capsule and observe that the iris ex¬ 
tends a little farther below the sclerotica than it appears 
from the front. When we direct the iris to strong light, 
after having washed off the dark pigment which covers 
the inside and produces its particular color, we are able by 
means of a microscope to detect the two sets of fibres, 
the circular and the radiating. 

After having cleaned the board, we take the second 
eye, which vjas kept in a glass of water, to finish this 
chapter with some additional remarks not yet explained. 
We cut from the back part, exactly opposite the pupil, 
a small piece of the sclerotica (the size of a gold dollar), 
and also of the choroid, watching carefully that the reti¬ 
na and vitreous humor are left uninjured. If we now 
encircle the eye by a piece of stiff paper, so that the pu¬ 
pil can be seen at one opening, and the retina at the oth¬ 
er, and direct the pupil to a well-lighted object, we see 
upon the transparent retina the small picture of that ob¬ 
ject in an inverted position. I will not attempt to ex- 


114 


HAND-BOOK FOR OPTICIANS. 


plain here, why we see everything erect and not inverted, 
as I did not find anywhere a satisfactory explanation of 
this phenomenon. But after all, it is of little moment 
to an optician to define something which is not yet fully 
explained by eminent scientists. I think, all writers on 
this subject overlook the simple fact that the retina is not 
the end-point of the act of vision, but that it merely rep¬ 
resents an inside lens of the series of lenses in a tele¬ 
scope. It is, therefore, immaterial in what position the 
object appears on the retina, as the brain really is the 
last factor in the act of seeing, the veritable ocular lens 
of our optical apparatus. 

After having unrolled the paper we place the eye on 
the dissecting board, pupil up, and remove the cornea 
and also the iris with the sharp-pointed scissors. If the 
cut is made outside of the iris we are able to lift the 
front center-part out of the ball together with the lens, 
and we see in the vitreous humor the cavity where the 
crystalline lens was imbedded. Those of my readers 
who are of an investigating proclivity and are in posses¬ 
sion of a strong magnifier will make the strange observa- 
tion, that the lens is not perfectly spherical on either 
side; the front part is elliptic, the back part parabolic, 
while the inside back of the eye, the retina, is spherical. 
Those well versed in mathematics will admit that the 
combination of these curvatures cannot be accidental, 
and that they must have been designed after a well cal¬ 
culated plan in order to work together harmoniously. 
— The more we reflect about this wonderful structure, 
the less proud we are of all our knowledge and learnings. 
How long did it take the human race to learn that little 
of this living camera obscura which we know to-day ! 
How many things are yet enveloped in a mysterious 
darkness ! Who, for instance, ever explained correctly 
the achromatism of the eye? Is it produced by the com¬ 
bined refractive and dispersive powers of the lens and 
the vitreous humor; or is it due to the different densities 
of the cornea and the lens; or perhaps to the dissimilar 
curvatures of the lens and the retina? Nobody knows. 
But notwithstanding of the many unveiled mysteries by 
which the eye is still surrounded, we must confess that we 


ANATOMY OF THE HUMAN EYE. 


115 


know more about the eye than we do of the functions of 
many other organs of our body; because, what we really 
know of it is based on mathematical principles relating 
to light and its refraction, while our knowledge of other 
organs are to a great extent still theories and conjectures. 


CHAPTER XVI. 


Presbyopia, Hypermetropia and Myopia. 


The general belief that the normal eye is perfect, is not 
true. For instance, the crystalline lens , the most essen¬ 
tial part of the refracting media, is far from being fault¬ 
less; it is not optically uniform, neither in shape nor in 
structure; its anterior surface is elliptically convex, and 
the posterior surface parabolically convex. Besides, 
the fibres are arranged around six diverging axes, so that 
the rays which we see around stars and other distant 
lights are mere images of this radiated structure. The 
statements of Alex, von Humboldt and Dr.E.Landolt, 
that they have known parties who could see the stars as 
luminous points, only show that these eminent scientists 
were simply imposed upon. There is also the retina; it 
performs its duty only on a limited spot, the “macula lu- 
tea”, which we call direct vision , applying the term in¬ 
direct to that exercised by the lateral parts of the reti¬ 
na. But all the defects, which result from the inexact¬ 
ness of vision, are compensated for by the rapidity with 
which we can turn the eye from point to point of the 
field of vision, and it is this rapidity of movement which 
really constitutes the chief advantage of the eye over 
any optical instrument. Helmholtz, while pointing out 
these and other defects of the eye, resignedly remarks: 
“I shall be only too glad to keep them as long as I can—de¬ 
fects and all.” Indeed, of all the members of the body, 
the eye has always been held to be the choicest gift of 
nature. Poets and writers have sung its praises; phil¬ 
osophers have extolled it as a crowning instance of per¬ 
fection in an organism, and opticians have imitated it as 
an unsurpassed model. The most enthusiastic admiration 
of this wonderful organ is only natural when we consider 
the functions it performs; when we dwell on its pene- 




EMMETROPIA 


117 


trating power, on the swiftness of succession of its bril¬ 
liant pictures, and on the riches which it spreads before 
our sense. It is by the eye alone that we know the 
countless shining worlds that fill immeasurable space, 
the distant landscapes of our own earth, with all the var¬ 
ieties of sunlight that reveal them, the wealth of form 
and color among flowers, birds and insects. Next to loss 
of life itself that of eyesight is the heaviest. But even 
more important than the delight in beauty and admira¬ 
tion of majesty in the creation which we owe to the eye, 
is the security and exactness with which we can judge by 
sight of the position, distance and size of the objects 
which surround us. For this knowledge is the necessary 
foundation for all our actions, from threading a needle 
to leaping from cliff to cliff, when life itself depends on 
the right measurement of the distance.—We, therefore, 
should not find fault with the few organic defects of the 
eye, as there are many others which appear to be partly 
the result of our artificial way of life, partly of the in¬ 
evitable changes of old age. 

The average good eye is called emmetropic , i. e. with¬ 
in measure, and ail defective eyes are called ametropic , 
out of measure. The meaning of in and out of measure 
can be best demonstrated by the following experiments. 
Take a convex lens of three-inch focus (+ 4), and a 
white card; mount each on a little stand, and direct the 
lens to an object twenty feet or more away, so that the 
rays reaching it are parallel. Then approach the card 
towards the lens till you have a sharp defined picture; 
when you now measure the distance between the card 
and the center of the lens, you will find it to be three 
inches , or exactly in “measure” (Emmetropia). 

We now place the card at two inches from the lens; 
instead of an image on the card we have a diffused patch, 
because the lens is out of focus, and only a two inch lens 
(_j_ would restore the picture. But instead of chang¬ 
ing the lens we only add to it the difference between + 4 
and + 4, which is +4; therefore, a convex Jens of six 
inch focus held before the test-lens will bring it again into 
measure ( Hypermetropia ). 

We remove the card how to four inches from the lens, 


118 


HAND-BOOK FOR OPTICIANS. 


and we have the same trouble; there is no picture but 
only a blurred patch, the focus of the lens is too short, 
and we have to lengthen it by the difference of 4 and 
which is i\-. As we can weaken a convex lens only by 
the addition of concave lenses, we are obliged to place 
— iV front of the test-lens in order to be again in 
measure (Myopia). 

This simple illustration shows why we have to correct 
hypermetropia b}^ convex, and myopia by concave lenses. 
—We also can make use of this experiment to explain 
Presbyopia and its correction by convex lenses. We put 
our apparatus again into the same position we had in 
emmetropia, i. e. the card is placed three inches from the 
lens, and we leave it there without further disturbance, 
only altering successively the strength of the test-lens. 
We first exchange -f 4 for + 1/34, and we find by calcula¬ 
tion that the difference between them is -f- 3 +, which we 
have to add to -f 1/34, to restore the strength of the orig¬ 
inal test-lens, in order to obtain a clear picture. Weaker 
lenses require stronger corrections; so is + 1/34 corrected 
+ 2 X; +v 1/3J by + -+ 5 ; + 4 by -PiV 5 + 1/44 by 
+ 4 , etc. 

If in the foregoing cases we substitute for glass-lens 
and card, crystalline-lens and retina, we have a fair ex¬ 
planation of the terms Emmetropia, Hypermetropia, My¬ 
opia and Presbyopia, respectively. 




PRESBYOPIA. 


119 


PRESBYOPIA . 

This defect of the human eye can be readily corrected 
by the use of convex glasses; it is due to the diminishing 
power of accommodation, and shows itself generally at 
the age of forty-five years, therefore called “old sight.” 
It is difficult for any one who has fair sight to realize, 
that seeing necessarily involves some muscular effort, 
unless it may be when looking at objects too near. 
Simply to open the lids seems all that is needed, which 
is practically true of real normal eyes; but we must re¬ 
member, that sight involves two distinct processes: first, 
the focusing of the light emitted from the objects looked 
at, so as to produce a clear picture on the retina; secondly, 
the perception or appreciation of this picture by the 
retina. The first is only an optical process, controlled 
and limited by the laws of optics: but the second is a 
physiological process of the optic nerve, and depends on 
its healthy action, which, fortunately, is the usual con¬ 
dition. 

The lens of the normal eye is just of the proper bending 
power to focus parallel pencils of light upon the retina. 
For nearer objects, there is a muscular arrangement for 
altering the curvature of the soft, elastic lens, so that it 
still keeps the focus upon the retina. These muscles 
give us the power of accommodation, or adjustment for 
varying distances; and for every minute alteration in 
distance, we have to make the corresponding, though un¬ 
conscious adjustment. Every one will find some point 
within which it is impossible to see clearly; this point is 
called the “near point,” and represents the full and 
utmost power of his muscles of accommodation. The 
crystalline lens in children is very elastic and easily acted 
upon by the ciliary muscles; but it soon begins to 
increase in firmness or hardness, so that the same mus¬ 
cular effort fails to produce the same amount of change 
in the refractive power of the lens, and the full action 
of the muscles fails to bring the near point as close to the 
eye as it used to do. 

Now, when the near point is about two inches from 
the eye, and the work held at eight or ten inches, there is 
a large surplus of reserve power , and the ordinary use 


120 


HAND-BOOK FOR OPTICIANS. 


of the eyes, on near work, is done with ease. But, as 
years go by, the near point recedes, and the eyes are 
obliged to call more and more upon their reserve 
power to accomplish the work they used to do easily. 
As long as the near point lies well within the working 
distance, vision can be used almost indefinitely, but when 
the near point has receded to nine or ten inches, one 
needs to get almost as near as that to his work, the 
accommodation is taxed to its utmost, and the strain and 
fatigue are very great, for no muscle can work at its full 
power long at a time. 

Since the progress of hardening of the lens and reces¬ 
sion of the near point goes on very gradually, there is 
no sudden change, no marked symptom to draw one’s 
attention. It will depend greatly, therefore, on the 
customary length of time the eyes are kept at work at 
close range, upon the general health, and supply of 
nervous force not otherwise called upon, whether and 
-when the eyes begin to suffer from old sight. This really 
begins quite early, but does not reach the condition of 
needing assistance until the near point has gone off to 
:about ten inches; it simply means overtaxing of the 
muscles of accommodation,—not on account of the work 
done, but changes in the effort required to do the work; it 
means improper wear and tear to the nervous system, 
just in proportion to the demand for near work. Although 
the use of glasses can be deferred for quite a long time 
after they are really needed, the cost is very real and 
has to be paid in some way. Just as machinery may be 
run for some time after oil is needed, and accomplish its 
work well, it is with a wast of power and damage to the 
parts. 

The moment we find that our usual work fatigues the 
eye, especially at night, it is proper to commence with 
spectacles, and + 0.25s will relieve us from undue strain. 
But when we do not listen to this first appeal to assist 
our failing eyesight, we are compelled to remove our 
work farther away, and only + 0.50s will enable us to 
see again distinctly at the proper distance. A further 
delay reveals the need of more light, we have to approach 
the window or door, or draw the lamp nearer to see well; 


PRESBYOPIA. 


121 


~\- 0.75s is now the correcting lens. — Up to this state 
of our eyes, we still can see, although with some difficulty, 
the finest print in the newspaper, and only a few people 
are aware of having already slighted three calls for cor¬ 
rection. Soon they will be surprised that they can no 
longer distinguish small print, or thread a needle, and if 
they now come for help we may relieve them with + Is; 
but many people will still defer the use of glasses, having 
been told to do without them as long as possible. They 
foolishly begin the vain struggle against the inevitable 
inroads of advancing age, and instead of growing old 
“ gracefully,” they resort to imprudent artificial means, to 
rejuvenescence; but as to their failing eyesight, there is no 
other remedy to resort to than the use of the “ dreaded ” 
spectacles. It is utterly useless to “fight age,” as far 
as our eyes are concerned, and the sooner we can con¬ 
vince people, especially ladies , of the absolute necessity 
now to commence with spectacles, the better for them. 
At this point, the optician can prove that he is something 
better than a simple mechanic, that he is also the scientific 
adviser in eye-troubles, which can be controlled by a 
judicious selection of glasses. 

The longer people are opposed to substituting by 
spectacles that part of their power of vision which is 
forever lost, the more rapidly their eyesight will fail, 
and we are sometimes compelled to hand to such incon¬ 
siderate persons glasses of + 2s, or even + 2.50s. They 
are for a while deceived by their splendid distant sight, 
but become alarmed as soon as they also perceive a failing 
of this last defense of their folly. They now feel sorry 
that they were badly advised, and have delayed the use 
of spectacles till their eyesight is injured beyond redress. 


HYPERME TR OP I A . * 

This defect of vision has its origin in a deformity of 
the eyeball. In presbyopia, the ball is practically a per¬ 
fect sphere, but its range of accommodation is impaired; 

* This word is derived from the Greek, hyper, beyond, metron, measure, 
opsis, vision, i. e., vision beyond measure; but some writers call it Hyper¬ 
opia,’omitting the word “metron.” This abbreviation is not intended 
to avoid the significant similarity between the above “long name,” and 




122 


HAND-BOOK FOR OPTICIANS. 


in hypermetropia (H.), the accommodation is good up 
to a certain period, but the optical axis is too short, so 
that parallel rays are not united upon the retina, which 
by its projected position intercepts them before they are 
focused, and it, therefore, receives only circles of diffu¬ 
sion instead of a clear image. 

Unlike presbyopia, which develops slowly and be¬ 
comes evident at middle age, this is a permanent defect 
in the shape of the eyeball; the retina is too close to the 
lens, and will naturally cause all its pictures to be more 
or less indistinct. But' as all eyes have the muscular 
power of increasing the strength of the lens, such an eye 
can, must, and does adjust the lens to the faulty position 
of the retina, and in that way sees perfectly well, but it 
has to use up more or less of its muscular power to get 
what the normal eye sees naturally and at rest. The 
normal eye is like the rower on smooth water, who util¬ 
izes his strength only for progress and rests at will. 
The hypermetropic eye is like one rowing up-stream, 
who must spend much of his force in overcoming the 
ceaseless current, and has only the balance for real pro¬ 
gress, and to him rest is impossible. It is, therefore, an 
overworked eye, not on account of what it does, but on 
account of its shape; for it has first to overcome its ever¬ 
present congenital defect, and in addition all the work of 
a normal eye as well. This constant, unavoidable strain 
shows itself sooner or later in some forms of fatigue or 
exhaustion. There need not be the slightest impairment 
of vision or pain in the eye, but inability to enjoy contin¬ 
ued vision without weariness, headache or inability to fix 
the attention long at a time; or there may be local symp¬ 
toms of blurring, smarting, or tired feeling in the eyes. 
Whether or not the hypermetrope experiences anj r of 
these symptoms will depend much on his general health, 
and the amount of close work done. He may escape 
them all his life, if strong and well; he may be made 
miserable in health by them even while in school. Very 
often, however, since the hypermetrope never is able to 

the “long time” it took the medical faculty to explain a defective con¬ 
struction of the eyeball,. which surely is as old as the human race; it is 
simply a verbal translation of the German word “ iibersichti» ” (over¬ 
sighted), and has the same meaning as Hypermetropia. 



H YPERMETROPIA. 


123 


compare his eyes with others, as to ease of seeing, he 
remains in entire ignorance that his sight costs him so 
much, and he may prove a constant patron of the phy¬ 
sician for tonics and assistance to combat his various 
troubles, or else settle down to the belief that he must 
endure his discomforts as part of his make-up. 

The deformation of the ball is either acquired , when 
in infancy the eye did not receive the proper amount of 
nutrition during the period of growth, and remained im¬ 
perfectly developed; or it is original , i. e. born with the 
child (congenital), and is often hereditary. The original 
H. is divided into manifest and latent H.; the remedy for 
them is a convex lens. 

In testing hypermetropic eyes for spectacles, we must 
select the strongest convex glass with which the patient 
can see Type XX at twenty feet. Young persons can 
use the same glasses also for reading, and should be ad¬ 
vised to wear them constantly if the defect is considerable 
or causes annoyance. Older persons may suffer in ad¬ 
dition to their H. from presbyopia, which compels them 
to use stronger glasses for close work, and weaker ones 
for distant vision. Old opticians remember with disgust 
the trouble they had with customers who never could be 
suited, and who exchanged their spectacles till they were 
either satisfied, or tired of trying anymore. Surely, 
their H. was not completely corrected by the spectacles 
selected for them, and they only found by repeated trials 
the approximate strength of glasses which gave them 
temporary relief. The cause of this trouble is, that 
hypermetropic persons, before using spectacles, tax their 
accommodation to excess, and when coming at last for 
glasses, they involuntarily employ a good deal of their 
accommodation from old habit, and consequently correct 
with the glasses they chose only a part of their visual 
defect. That part of H. which they really corrected is 
called manifest H., and that part made up by force 
of accommodation, and which was not corrected, is termed 
latent H. 

We are here confronted by a powerful foe, who bathes 
our greatest efforts. The old trick of telling people 
that their eyes will “ accustom ” themselves to the use 


9 


124 


HAND-BOOK FOR OPTICIANS. 


of spectacles we selected, is played out, and we cannot 
any longer degrade our vocation to the level of a bung¬ 
ling shoemaker, who consoles his trusting victim with 
the assurance that in a few days the shoe will not pinch 
anymore. Remember, that the right glass relieves the 
eye forth-with; but in case we cannot find this glass, we 
should send the customer to an oculist, who will over¬ 
come all latent H. by the application of a mydriatic, 
which enables him to readily determine the strength of 
glasses, which will correct the entire or absolute H. It 
is very tempting for opticians to try the same thing, but 
I seriously warn them not to overstep their limited 
sphere, and frivolously invoke “ a sea of troubles ” by 
the application of drugs, which may, for instance, in 
Glaucoma, injure the eye to such an extent as to involve 
them in a costly lawsuit. Even those opticians, who are 
in possession of a Diploma from an Optical College, are 
not protected by it from suits for damages in case of an 
accident. 

Hypermetropiahas often been confounded with myopia; 
this generally occurs to persons under twenty years of age. 
The reason for this mistake is obvious; young hyper- 
metropes hold the book close to the eyes to get the larg¬ 
est retinal image, which again causes the pupils to con¬ 
tract and cut off the circles of diffusion, and also incites 
the ciliary muscle to make spasmodic efforts to increase 
the convexity of the lens, so that parallel rays may be 
focused even in front of the retina, thus simulating 
myopia. By this exertion the eye will incur frequently 
the troublesome defect of seeing objects double (diplopia ). 
To avoid this annoyance, the child often adopts the habit 
of inclining the head so that one eye is shaded by the 
nose, and only the other is employed. The consequence 
is that the unemployed eye gradually converges, and 
produces convergent strabismus or squint. 

A father brings his hypermetropic son to us, stating 
that he is near-sighted, because he holds the book very 
close to the eye, and always complains that his eyes 
hurt him, or are full of tears. Ignorant opticians may 
agree with such an unprofessional diagnosis, and give 
him concave spectacles, at the same time instructing the 


HYPERMETROPIA 


125 


parent to compel the child to wear them constantly, as 
the eyes soon would “ accommodate themselves ” to their 
use; thus rendering this poor child a lamentable victim 
of our ignorance. But, not only the parents and incom¬ 
petent opticians were in error about the real nature of 
this peculiar deficiency, also the medical faculty was 
ignorant about it, till Donders, Helmholtz, and many 
others afterwards, lifted the veil and explained the 
whole trouble. 

Hypermetropia may be easily detected by testing how 
far off one can read ordinary print through a lens of 
known focal length; for instance, if one looks through 
a ten-inch lens and can read clearly at a greater number 
of inches, say twelve, fourteen, or more, then he surely 
is far-sighted. 

It is by no means necessary for every one who is 
hypermetropic to wear glasses, for, if he experiences no 
signs of nervous trouble or over-fatigue, it is perfectly 
safe to leave the matter alone, although theoretically he 
needs help. But the recognition of H. should put one on 
his guard, and would supply a positive diagnosis for many 
forms of nervous difficulty which might arise, and explain 
many forms of fatigue and disturbance liable to be laid 
to the brain, the stomach, the liver, etc. Glasses for the 
hypermetrope simply take off the constant burden at 
will, do the extra work for him, and give him as fair a 
chance as normal eyes have. They save his accom¬ 
modation for its proper use, and should be worn enough 
to make his vision comfortable. 

The way of dealing with such cases is very simple; 
the statement of being unable to see distinctly at long 
distance, is not any longer taken as a proof of myopia, 
because the next test, to read small print, will show that 
we have before us a clear case of hypermetropia. 

Let me finally draw the attention of the reader to the 
frequent occurrence of styes on the lids of a hyperme¬ 
tropic eye. Before using spectacles, and also afterwards 
when latent H. is not fully corrected, the excessive 
effort of accommodation draws more blood to the eye 
and to the neighboring parts, than is necessary for their 
nutrition; the eyelids become swollen and sties are the 


126 


HAND-BOOK FOR OPTICIANS. 


final result. Dr. Soelberg Wells (1873) speaks of them 
as a disease of the connective tissues of the lids, for 
which he recommends cold compresses, and if they are 
without effect, hot poultices. He then orders a small 
incision to be made, and to prevent a recurrence of the 
disease, to apply a weak ointment of nitrate of silver. 
But if the patient is feeble and out of health, tonics 
should be given, and the digestive functions thoroughly 
regulated. — I believe, Dr. Wells will laugh to-day at 
his antiquated treatment of this disease , and will order 
suitable spectacles , instead of poultices and purgatives. 


MYOPIA. 

This deficiency is also described by most writers as a 
disease of the eye, which, in their opinion, gives rise to 
“posterior staphyloma” (an extensive bulging of the 
back portion of the globe), or to spasms of the ciliary 
muscle, etc. Although those symptoms are sometimes 
accompanied by myopia, yet they are no more caused by 
it than the softening of the bones by bow-leggedness; 
there is evidently a confusion of cause and effect. In 
perusing some of the best authorities on Ophthalmology, 
I have regretted that none of them are near-sighted, and 
have treated this subject from their own experience; it 
would have greatly modified some of their pet-theories 
about this “disease.” We also find in every text-book 
the traditional error of an undue prominence of the cor¬ 
nea in myopia, although Helmholtz and Donders have 
distinctly disproved the general truth of this statement by 
laborious measurements of the cornea and by the ophthal¬ 
mometer. For the consolation of myopes, I will treat this 
subject independently of the authorities, guided partly 
by the experience of my own myopic eye, and partly by 
the long practice as a dispensing optician. 

Myopia is a gift from Pandora’s box of civilization, 
as this defect of sight is not known among uncivilized 
nations, or in ancient times when people did not use their 
eyes on small objects, and in artificial light. It is more 
frequent since the invention of printing, and of improved 



MYOPIA 


127 


lamps and lights. Myopia is always artificially acquired, 
and a tendency to it is then transmitted to the children, 
although some children of myopic parents do not inherit 
this debility, but enjoy in youth and manhood perfect 
emmetropia. Since we are able to correct this deficiency 
with glasses, the myopes may be well satisfied with their 
splendid near-sight, and should not grumble at the need 
of glasses for distant vision, as this inconvenience is fully 
balanced by their perfect sight at short distance, even 
to old age. 

The names of the former two deficiencies, presbyopia 
and hypermetropia, indicate exactly the nature of their 
trouble, but this is not the case with the word myopia, 
which only means “blinking,” from the habit of all 
near-sighted persons to partially close the eyelids, (pro¬ 
ducing a stenopaic slit), in order to lessen the circles of 
diffusion and improve distant vision. Still, this word is 
generally accepted, and does not frighten people half as 
much as when we tell them they are suffering from 
brachymetropia, as Donders proposes to call it, or when 
we mention even the harmless hypermetropia. Our Eng¬ 
lish word short-sightedness is far more expressive, and is 
understood by everybody. 

The experiments with the lens and card at the begin¬ 
ning of this chapter illustrates only one kind of myopia, 
(M), called axial M.,because the optic axis is too long, 
which is always present in pronounced cases of M. But 
there are many mild cases of this deficiency, where the 
eyeball is of a normal shape, yet an abnormal con¬ 
vexity of the crystalline lens causes short-sight to a cer¬ 
tain degree, termed refractive M. Nearly all children 
are born myopes or hypermetropes; but their M. is only 
refractive, their lens is so convex, and on account of its 
great flexibility, so easily rounded and adjusted, that the 
focal distance of the eye is just in proportion to their 
diminutive size of body and their short arms. I recol¬ 
lect, that at the age of six years, I could see distinctly 
at nose’s length, although my M. was at the age of fif¬ 
teen only - f r . 

During'youth, the refractive M. may become axial, 
when there is a hereditary pre-disposition to it, combined 


128 


HAND-BOOK FOR OPTICIANS. 


with some local causes, as congestion of the ciliary mus- 
cles and other tissues, which sometimes leads to soften¬ 
ing and elongation of the eyeball. This congestion may 
be°produced by general mal-nutrition of the body with 
an excessive use of the eyes upon fine objects, by 
insufficient light or in stooping position. The foundation 
of axial M. is always laid in childhood. What is said of 
progressive M., is often due to the great difficulty in test¬ 
ing the eye, or to the careless way in which the eyes of 
myopic children are tested. If one needs — -§-» and we 
hand him — tV, he may be temporarily satisfied with the 
partial improvement in his former poor distant sight; 
and when we give him afterwards — J, he is more 
pleased, till we reach the full correction — Now, 
when this loose manner of correcting M. took us three 
years, we cannot well speak of progressive M., as the 
progress in this case is only on our side in correcting 
gradually former mistakes. 

But even grown persons may be ill-suited for years 
without the slightest suspicion that their eyesight could 
be considerably improved by stronger glasses. I had a 
lady-customer who asked always for — One night I 
met her at the theatre with glasses of a different pattern 
from those she generally bought of me; and as I was well 
acquainted with her, I was allowed to examine them, and 
found them to be — J. When I asked her, why she had 
never before complained of her insufficient sight, and 
had always called for weaker glasses than she really 
needed, she remarked that an oculist had prescribed that 
number for her, but that she had lately consulted Dr. P. 
(a peddling oculist),who had furnished her these splendid 
glasses, (by the way a six-dollar-glass), for only twenty- 
five dollars. She was well pleased, and could see now 
“the show” better than ever before. — Similar cases 
may have led our authorities to exaggerate the optical con¬ 
dition of the myopic eye, especially when they found* 
after a thorough test, that the old glasses were too weak, 
although they never before may have suited the case. 
Hartridge says: “ The higher degrees of M. which in¬ 
crease steadily and constantly from an early age, reach¬ 
ing often a "high degree, and carrying in their wake 


MYOPIA 


129 


destruction and damage to important ocular tissues, must 
be looked upon as a ‘ serious disease’; it is designated by 
the name malignant or progressive myopia.” My lady- 
customer could have been counted among the progressive 
myopes as long as she was wearing too weak glasses; but 
since they were properly corrected, her M. is no more pro¬ 
gressive; on the contrary, she is at present, (after the 
lapse of twelve years since that episode), well suited with 
— 1/4J. Nearly all authorities are recommending the 
weakest concave lenses for myopes, but the strongest 
convex lenses for hypermetropes; why ? I do not find any 
reasonable answer in all their arguments.* If they are 
so particular in correcting the absolute hypermetropia, 
why shall we not correct the absolute myopia, the more 
so, as the powerful accommodation of a myopic eye indi¬ 
cates an immense amount of latent M., of which no opti¬ 
cal writer seems to know anything. 

I wish the foregoing not to be construed as if I were 
© © 

opposed to that important and well-established rule, to 
select the weakest glasses with which a myope can see 
the test-type XX at twenty feet. On the contrary, 
I am strongly in favor of even making a kind of com¬ 
promise between distant and near glasses, as most 
myopes have the habit of wearing their glasses from 
morning to night. Hartridge says: “In young people 
with good accommodation and with a low degree of myo¬ 
pia, the full correction may be well borne, the patient 
wearing such glasses constantly; and it has been ob¬ 
served, that in those, who from youth have worn their 
full correction constantly , for both near and distant 
objects, the myopia has usually remained stationary.” 
This statement is not far from being correct, but unfort¬ 
unately, he forgot to mention that the eyelids of most 
of those myopes are often reddened and swollen from 
the undue strain of close work with spectacles of full 
correction. If we cannot induce the patient to buy two 
pairs of spectacles, one for each purpose, then let us 

* “The weakest glass with which Y = 20/xx is chosen, because the 
myope under 45 often “puts on” accommodation on looking at the test- 
types, and so makes himself seem more nearsighted than he is, and if we 
are not careful to give the weakest glass, just barely giving 20/xx, we 
make him into a hypermetrope. ” — De. H. D. Bbuns, New Orleans. 



130 


HAND-BOOK FOR OPTICIANS. 


adopt the practical method of a sensible parent, when 
buying clothing for himself and for his growing boy. 
His body has attained the full growth, is stationary, or 
even may by degrees shrink and fall off; he represents 
the myopic eye, and, therefore, should select a rather 
close fit. But how would a selection on the same prin¬ 
ciple do for his growing son, who represents the hyper¬ 
metropic eye ? Would he not out-grow his suit in no 
time and require a larger one ? 

The most sensible writer on this subject is Dr. E. 
Landolt; he says: “ You will find, in nearly all treatises 
on ophthalmology, myopia described as a serious disease 
which is liable to bring about choroiditis, alterations at 
the macula, staphyloma posticum, and even choroidal 
hemorrhages, and detachment of the retina. Properly 
speaking, myopia is not a disease, it is only a symptom 
indicative of a discrepancy between the length of the eye 
and the focal distance of its dioptric apparatus. It is 
not the myopia which produces the choroiditis and 
staphyloma posticum, which in its turn removes the 
retina beyond the focus of the dioptric system. Thus 
the defenders of the theory, generally accepted in regard 
to myopia, will be much embarrassed when they are 
shown a hypermetrope with a crescent at the edge of the 
optic disc, a papilla obliquely placed, and even with 
staphyloma; in a word, with all the conditions at the 
fundus of the eye which are theoretically characteristic 
of myopia. In fact, no eye is safe from an attack of 
choroiditis posterior; the emmetropic and hypermetropic 
eye can be affected as well as the myopic, because it is 
not the state of refraction that is the cause of it.” We 
see this plainly in cases of ‘Second Sight’, of which I 
speak in another chapter. 

There is another error, regarding the action of concave 
glasses on myopic eyes, found in all text-books and ac¬ 
cepted in good faith by most oculists and opticians, i. e. 
that concave glasses apparently diminish the size of 
objects and letters. This is a mistake. We have seen 
that in refractive M. the eyeball is of the same regular 
shape as in the emmetropic eye, and that the only differ¬ 
ence between them is the greater convexity of the crys- 


MYOPIA 


131 


talline lens in the myopic eye. Now, when we express 
the visual power of the emmetropic eye by + Is, and 
that of the myopic eye by + 2s, we must admit that all 
myopes see things at their focus really larger than the 
emmetropes see them, and when we put — Is before the 
myopic eye to correct the focal length, we only bring 
their sight to the standard size, and actually have not 
diminished the real size of the objects. A myope gener¬ 
ally answers, when asked if he sees through the glasses 
the objects diminished: “No, everything looks larger 
to me,” a common deception with children who confound 
the present clearness with the former indistinctness, 
being entirely incapable of judging the real size of 
things. Of course, an emmetrope will see things smaller 
through any concave lens, because his crystalline lens has 
not to waste any surplus of convexity, as we know is the 
case with all myopes, who on this account do not perceive 
the least shrinkage in the size of any objects by the use 
of suitable concave glasses. 

There is another peculiarity in the myopic eye which 
has not yet sufficiently attracted the attention of optical 
writers; it is the small handwriting of all myopes. 
Although this habit is contracted by them before they 
ever used spectacles, we still find this practice to a good 
extent after we have partly corrected their near-sighted¬ 
ness according to professional rules. In my opinion, 
M. is the least explored deficiency of all ophthalmic errors, 
but I hope that scientific men will make it a special study, 
and soon benefit the world, as Donders did, in regard to 
H., with a more correct treatise on myopia. We surely 
have to take in consideration what we may term latent 
M., otherwise we are at a loss to explain the small hand¬ 
writing of myopes who wear spectacles.* If there is 
also absolute M., let us fully correct it, as we have 
seen it done in H. The future investigator has to drop 
at once the erroneous opinion that a myopic eye, bring- 

* “Myopes write small because when, young the writing held at the 
focal distance of their eyes looks to them sufficiently large, — the habit 
formed when learning to write is never abandoned. Besides the use of a 
correct (not too strong) glass does not materially diminish the size of the 
object. That mainly depends on the distance of the lens from the screen, 
(retina). ” — Dr. H. D. Bruns. 



132 


HAND-BOOK FOR OPTICIANS. 


ing everything near to the eye, is more strained as to its 
accommodation than the emmetropic eye is by reading 
at a distance twice or three times that length. Watch¬ 
makers who necessarily use a strong convex lens before 
one eye, thus making themselves artificially near-sighted, 
and work at the focal distance of that lens, are not liable 
to-become myopic; proving that close work without con¬ 
vergence does not tend to produce myopia. The profes¬ 
sional microscopist is another illustration of this fact. 
As long as a myope is not compelled to hold the book 
nearer than six inches, the recti interni are still able to 
center both eyes for near work, though we must admit 
the strain to be considerable; but when the focus of the 
eye is shorter than six inches, the myope will soon form 
the habit of using only one eye, and divergent squint will 
be the consequence, if his M. is not corrected for near 
vision. This is especially the case with myopes under 
twenty years. 

The phenomenon that some persons are presbyopic 
and myopic in the same eye, needs a short explanation.* 
The text-books call this state of vision “ Myopia in 
Distans,” or short-sight at distance, but they do not ex¬ 
plain what changes in the eye have taken place to pro¬ 
duce this phenomenon. Donders thinks that it is often 
due to abnormal dilatation of the pupil, and Yon Graefe 
attributes it to a peculiar spasm of the ciliary muscle 
during the attempt of relaxation in adjusting the eye for 
distant objects, but both explanations do not unveil the 
cause of the “dilatation” or of the “spasm,” which 
surely must be due to certain defects in the mechanism 
of the e 3 ^e. Before I offer my own theory on this inter¬ 
esting deficiency, let me correct the general error that 
the normal eye is in a state of absolute rest when it is 

* “ An eye can only be myopic and presbyopic when tbe myopia is less 
than 2 or 2.50 D ( i . e. the focus longer than 16 or 18 inches), and the 
patient 40, or prematurely old and feeble in accommodative power. I 
have sought in vain for evidence of “negative accommodation” ( i . e. 
power to flatten the lens). I never have seen a myope who could by effort 
reduce his true myopia, or a hypermetrope who could increase his defect. 
In the case above mentioned, a presbyope who had M — 2 D, would have 
to use a convex near glass, or hold his reading off the ridiculous distance 
of half a meter (20 inches). Of course, he would need — 2D for good 
distant vision.” — Dr. H. D. Bruns. 



MYOPIA 


133 


adjusted to bring parallel rays to a focus upon the retina. 
The far-point as well as the near one necessitates an 
effort of the accommodation, and the point of absolute 
rest lies consequently between the two. Accommodation 
is produced by the action of the ciliary muscle, which, 
similar to the ciliary processes (the iris), consists of two 
different sets of fibres, the circular and the radiating. 
The contraction of the circular fibres adjust the lens for 
near vision, and the contraction of the radiating for 
distant vision. Their action is antagonistic; the con¬ 
traction of one set of fibres relaxes the other, and only 
when both sets are in a state of absolute rest, distinct 
vision ceases , we are gazing into vacancy, and feel the 
change of the tension in the eye the moment we try to 
focus for an object either near or far. There are two 
ways to explain the above deficiency: either the ciliary 
muscle has lost some of its power to properly shape or 
adjust the lens for different distances, though the lens 
may be still in its normal state; or, if the muscle has 
retained its full power, the lens may have lost some of 
its flexibility in getting harder and more rigid, thus offer¬ 
ing greater resistance to the action of the muscle; or 
there may be a combination of both deficiencies. In all 
cases, neither the contraction nor the relaxation will be 
completely performed; the muscle cannot reach its 
former extreme points of accommodation, — its action 
resembles the shortened vibrations of the hair-spring of a 
watch whose main-spring is almost run down. For near 
vision the lens is not enough rounded up to dispense w T ith 
convex glasses, and for distant vision it is not sufficiently 
flattened to do without concave glasses. 

It is immaterial for opticians to further investigate the 
question, which of the two deficiencies in the accommo¬ 
dation of the eye is more frequent, the loss of muscular 
power or the hardening of the lens; although a little 
reflection may lead us to the conviction that the muscular 
debility occurs more frequently in earlier life, and that 
the hardening of the lens is mostly confined to old age, 
especially when “second sight” fore-shadows greater 
trouble. 


CHAPTER XVII. 


Astigmatism. 


From the beginning of my studies in optics to the pres¬ 
ent day, the word astigmatism has always impressed me 
with a devout feeling; and at this very moment, as I write 
about it, I feel as if I had entered a temple of worship, 
and were listening to a hymn of praise to the great achieve¬ 
ments of the human intellect. History glorifies the vic¬ 
tories of brave soldiers; but their deeds spread woe and 
misery among thousands of their companions, while the 
victories of scientific investigators create rejoicing and 
happiness. The researches in astigmatism are, indeed, 
the crowning cupola of that magnificent structure, 
“ Ophthalmology.” Although its beginning dates back a 
whole century, its completion is the work of the last thirty 
years. Science generally progresses slowly; the first ele¬ 
ments of it appear in the form of isolated facts. As 
these multiply, a kind of mutual connection appears pos¬ 
sible. Possibility becomes successively probability; 
probability, certainty. And thus the individual truths 
of science, like the wheels and pinions of the engine, be¬ 
come all subservient to one great common end. In no 
branch of science has this been better exemplified than 
in our knowledge of the modifications of the refraction 
and accommodation of the eye. 

The first discoverer of this peculiar defect of the eye 
was Thomas Young, in 1793. His eyes, when in a state 
of relaxation, collected to a focus on the retina those rays 
which diverged vertically from an object at the distance 
of ten inches from the cornea, and the rays which diverged 
horizontally from an object at seven inches. Consequently, 
the refraction of his globe was stronger in the horizontal 
than in the vertical meridian. In 1827, Professor Airy 
published a remarkable instance of the same anomaly in 




ASTIGMATISM. 


135 


his left eye. In this, the furthest point of distinct vision 
for vertical rays was three and a half inches, and for hor¬ 
izontal ones, six inches ; the eyeball thus being nearly 
twice as myopic in the vertical than in the horizontal me¬ 
ridian. To Airy likewise belongs the merit of first hav¬ 
ing applied cylindrical lenses for the correction of astig¬ 
matism. In Young’s case, the astigmatism originated in 
an irregularity of curvature or position of the crystalline 
lens, therefore, called lenticular astigmatism, while Airy’s 
deficiency was due to an imperfection, in the curvature of 
the cornea, called corneal astigmatism. When Donders, 
in 1862, published his work on astigmatism, only eleven 
cases of this optical defect had been recorded, but he states 
that astigmatism is a very common disturbing cause of 
vision, and that many cases hitherto but imperfectly cor¬ 
rigible by spherical lenses, are almost completely so by 
cylindrical ones, either alone or combined with spherical 
ones. —To-day we may count the cases which are suc¬ 
cessfully corrected by cylindrical lenses, by the million. 

Before we go into details, it may be proper to remind 
the reader of some peculiarities, present more or less in 
every eye. The average eyeball is considered to be a per¬ 
fect sphere, but this is not mathematically true, — on ac¬ 
count of the cornea. What we see of the eyeball, when 
the lids are open, is not a circle but an ellipse; this is the 
reason why our field of vision is laterally fully 160°, and 
vertically only 120°. The large lateral scope of vision 
may be the cause of the cornea being somewhat flattened 
in the horizontal meridian, by the constant pressure of the 
edges of the eyelids, while in the vertical direction this 
pressure is very slight. If we take an egg, or the bowl of 
a spoon, and draw a line from point to point, it will repre¬ 
sent the horizontal meridian of the eye, and the line 
across the middle will be the vertical meridian. Of 
course, each of these meridians has a different length of 
focus; the vertical is more convex and will concentrate 
the rays to a shorter focus than the horizontal; and to 
correct this deficiency we have either to lengthen the fo¬ 
cus of the vertical meridian, or shorten that of the hori¬ 
zontal one. Spherical lenses cannot do this, because any 
shortening or lengthening' would be equal in both merid- 


136 


HAND-BOOK FOR OPTICIANS. 


ians; only in cylindrical lenses have we the means of per¬ 
forming this feat. According to Chap. IV, the cylindri¬ 
cal lens is a plane in its axis, and only at right angle, or 
ninety degrees from the axis, does it act as a spherical 
lens of the same denomination. Prof. Airy, for instance, 
had to lengthen the focus of his vertical meridian by 
a concave cylinder, axis 180°, and if we suppose that his 
right eye had a focal distance of ten inches, he then had 
to combine the cylinder with the proper spherical con¬ 
cave lens, to equalize the focus of the left and right eye. 

A common cause of astigmatism is that the cornea and 
the crystalline lens are not symmetrically placed with re¬ 
gard to their common axis, they are not accurately cen¬ 
tered. This defect is found in most human eyes, but is 
perhaps corrected, in mild cases, by an irregular contrac¬ 
tion of the ciliary muscle, in the same way as we invol¬ 
untarily adjust the center of gravity of our body by stoop¬ 
ing forward when we carry a heavy load on our back. 

Astigmatism is sometimes congenital; but in most 
cases is mechanically produced by injuries, wounds, 
ulcers, etc., or is due to the pressure of swollen lids upon 
the cornea or to sties, which are often met with in hyper¬ 
metropic eyes; wherefore these two deficiencies are 
frequently combined in the same eye, called hyper¬ 
metropic astigmatism. If a myopic eye is thus affected, 
we call it myopic astigmatism. — A fruitful source of 
corneal astigmatism was patented in 1867 by Dr. E. B. 
Foote of N. Y., called the “Eye Sharpener.” This 
physician entirely ignored the organic changes which 
take place in the eye by age, as we see by the first lines 
in his circular: “It is pretty generally understood that 
the reason why people advancing in age are compelled to 
hold the work or newspaper farther from the eye than 
they were accustomed to do in youth, is because the 
eyeball has become flattened.” He, therefore, invented 
a sucking-contrivance in the shape of a cup, “to keep 
up the fullness of the cornea,” and attached to it a 
depressing-device to flatten the cornea of myopes. Some 
of my customers were lured into the meshes of this 
ignoramus to their great sorrow; let me give you an 
instance. A prominent lawyer in N. O. was near-sighted 


ASTIGMATISM. 


137 


to the extent of — Js or 5 Ds; these glasses gave full 
satisfaction for many years. One day he asked for 
— -J-s, then for — till he called for — all in 
one week; but he soon returned to almost the same 
number he started from; his apparent improvement 
was nothing but a grave illusion. For several years 
after this, I lost sight of him, perhaps he consulted an¬ 
other optician; till lately he handed me an order from 
an oculist for — 5s Q — 2c axis 90°, which was the 
final result of his previous experiments with the Eye 
Sharpener. 

Astigmatism is divided into three varieties: 

2. Simple hypermetropic and myopic astigmatism. 

2. Compound “ “ “ 

3. Mixed astigmatism. 

All these forms are called regular , while the existence 
of different degrees of refraction in one and the same 
meridian is termed irregular astigmatism. It is not 
necessary for a practitioner to be thoroughly acquainted 
with the many technical terms used in this respect. 
Landolt says: “In the vast majority of cases, fortu¬ 
nately, it suffices to know the total astigmatism of the 
eye, without questioning ourselves as to what part is due 
to the cornea and what part to the crystalline.” — All 
books treating of astigmatism are written by physicians 
for physicians; their writing is, therefore, too much in¬ 
terlarded with Latin and Greek, that it is all Greek to 
the plain optician, who has not had the benefit of a scien¬ 
tific education. But such terms are sometimes very handy 
in technical explanations, partly for brevity, partly for 
exactness of expression. The word astigmatism is one 
of them; stigma means a point, and astigma , no point, 
i. e., the rays of light are not uniformly united on the 
retina to a point or focus. This is also the case with all 
other ametropic eyes, but they can be easily corrected 
by spherical glasses, which are of no value to correct 
astigmatism. 

To discover astigmatism, several devices, such as a 
fan, or a dial, have been introduced; but I found Dr. 
Pray’s striped letters most convenient for indicating one 
of the chief meridians. They can be used by every one, 


138 


HAND-BOOK FOR OPTICIANS. 


whether he can read or not, because it is only necessary 
for the patient to state which letter is the blackest. Dr. 
Owen, recently, improved upon Pray’s design by pub¬ 
lishing a card comprising two sets of letters, each one 2i 
inch square, formed of lines which radiate towards every 
ten degrees, in order to find readily any faulty meridian 
from 10° to 180°. With the assistance of the improved 
trial-frames and a complete set of lenses, it is not diffi¬ 
cult to correct most cases of simple and compound astig¬ 
matism. — The simplest case is when one meridian is 
still emmetropic; we test such an eye with Snellen’s 
test-types. The patient will ^tate that he sees type XX 
quite well, but that a continued gaze at them causes a 
heavy feeling in the eye, which soon will be followed by 
headache. We then take a convex and a concave lens 
of 0.50s, one in each hand, and place.first one, then the 
other before his eye; if he does not find any improve¬ 
ment with either of them, then a quick glance at Pray’s 
letters will disclose that some of them are blacker than 
others. We now take the trial-frame with either -j- or 
— 1.00c, the axis placed at right angle to the blackest 
stripes he before had pointed out. The lens which im¬ 
proves his sight is taken as a guide in trying if stronger 
or weaker ones will still give better satisfaction. It is 
necessary to try both, convex and concave cylinders, as 
we do not know if we have to shorten or lengthen the 
faulty meridian. But, where is the faulty meridian? 
Seemingly in that direction where we see the lines pale 
or indistinct; yet, this is an error, or an optical delusion, 
as the faulty meridian is just ninety degrees from it, or 
in the direction of the blackest lines. "To explain this 
paradox, I have to remind you of the peculiar propagation 
of light by waves. A horizontal ray is propagated by 
waves which move up and down, vertically, or at right 
angle to its direction; in the same way is a vertical ray 
produced by horizontal vibrations. Suppose we take 
some thin slips of paper and pile them up to a column, 
then the slips will lay horizontally, but the column will 
be vertical, and to build a horizontal line we have to 
place the slips vertically. When we, therefore, look at 
Pray’s letters and find those of the vertical stripes the 


ASTIGMATISM. 


139 


blackest, we see them with the horizontal meridian of our 
eye; and if the horizontal stripes are the blackest, we 
perceive them with our vertical meridian. The faulty 
meridian, therefore, lies always in the direction of the 
blackest line, and we have to put the axis of the cylinder 
ninety degrees from it. 

Sometimes the faulty meridian of one eye is at rio*ht 
angle to that of the other eye, when in binocular vision 
they will correct each other; it is, therefore, absolutely 
necessary to test each eye separately. This kind of 
astigmatism is called simple hypermetropic or myopic 
astigmatism , according to the nature of the correcting 
cylinder, which will be either plano-convex or plano¬ 
concave. 

The second variety of astigmatism is the combination 
of astigmatism with hypermetropia or myopia. Its cor¬ 
rection depends entirely upon the relative proportion of 
each deficiency. Prof. Airy, for instance, had to correct 
his astigmatism before he could equalize the focal distance 
of his eyes by the addition of concave spherical lenses. 
But if hypermetropia or myopia is in excess of the 
astigmatism, we better correct them first in the usual way, 
and finish off by adding the correcting cylinder. It 
happens sometimes, after we have evidently corrected 
the full amount of hypermetropia or myopia, and added 
the cylinder, with axis at right angle to the direction of 
the blackest line, that we have to turn the cylinder ninety 
degrees, in order to get clear vision. This strange 
incident could be called apparent astigmatism, although 
there.is no such thing, because the double nature of 
spherical lenses, being crossed cylinders as well as seg¬ 
ments of spheres, will easily explain it. Let me refer 
to my own myopic-astigmatic eye, which sees the hori¬ 
zontal lines the blackest; the faulty meridian, therefore, 
is at 180°, and the lens which is to me most satisfactory 
and pleasant, is — 2s Q — lc axis 90°. But it would be 
an excellent illustration for the theory of apparent 
astigmatism had I corrected my myopia by — 3s, and 
then the apparently faulty meridian by + lc axis 180°. 
When we look at the two combinations: 

— 2s Q — lc axis 90°, and 

— 3s C + lc “ 180°, 



140 


HAND-BOOK FOR OPTICIANS. 


we will find them to be equivalents, only my eye prefers 
the biconcave to the periscopic form. If, therefore, the 
axis of the cylinder has to be placed in the direction of 
the apparently faulty meridian, instead of ninety degrees 
from it, we have simply over-corrected the hyperme- 
tropia or myopia of the eye, and have made an error in 
the selection of the spherical lens. — The lenses which 
correct compound hypermetropic and myopic astigmatism 
are called Compound Lenses. 

The third variety of astigmatism cannot be well cor¬ 
rected without the application of a mydriatic, and is, 
therefore, beyond our reach. 

The irregular astigmatism even eludes the greatest 
efforts of the expert oculists, because we opticians can¬ 
not produce the suitable lens for its correction. 


CHAPTER XVIII. 


The Ophthalmoscope. 


Prior to the invention of the ophthalmoscope it was 
impossible to explore the interior of a living eye. Scien¬ 
tific men tried in vain to interpret this strange fact, for 
apparently this should have been easily accomplished, 
since the depth of the eye is shallow and all the struct¬ 
ures situated in front of the retina are transparent; yet 
it proved to be a conundrum, until Helmholtz explained 
the whole mystery. 

The optical law that the angle of reflection is eoual to 
the angle of incidence had been known two thousand 
years. Polished surfaces and the bottom of shinino' 
vessels with wide openings were used to demonstrate 
its correctness; but when the angle of incidence became 
so small as to be almost, or completely, parallel to the 
angle of reflection, as is the case when light enters the 
eye and should be visibly reflected through the pupil, 
this law was entirely overlooked, and many theories were 
formulated to explain the black appearance of the pupil. 
A faint luminosity of the eye, especially in the tapetum 
of dogs and cats, had been observed from the earliest 
times, and gave rise to the general believe that the human 
eye was also luminous under certain conditions. With a 
kind of popular superstition it was regarded as evidence 
of a voluntary nervous irritation on the part of the ani¬ 
mal, although nothing of the kind could be perceived in 
the human eye. 

The first who opposed this general error was the Ara¬ 
bian, Alhazen, and afterwards the Italian, Battista Porta, 
but their protest was not noticed; on the contrary, when 
Mery, 1704, observed the retinal vessels of a cat under 
water, the old superstition was strongly revived, till in 
1810, Gruithuisen and Prevost repeated the very experi- 



142 


HAND-BOOK FOR OPTICIANS. 


merit in the dark, and of course did not see anything. 
They, therefore, denied the self-luminosity of the eye, 
and referred the aforesaid phenomenon rightly to reflected 
light. This was the end of the old delusion, and it was 
reserved to our century to solve the question: why is the 
pupil black ? 

In 1846, Dr. Cumming published a paper “On the 
Luminous Appearance of the Human Eye,” and rarely 
has an observer approached closer to an important dis¬ 
covery without actually reaching it. The next year, 
Dr. E. Briicke published the account of an experiment 
in which he allowed the light from a lamp to enter the 
observed eye, whilst he approached his own eye very 
closely to the flame, only protecting it from the glare and 
heat by a thin screen. Both, Cumming’s and Briicke’s 
principle was for the observer to regard the eye in a 
direction nearly parallel to the entering rays of light. 
But Helmholtz, in 1851, was the first >vho clearly per¬ 
ceived the true optical relation between the incident and 
reflected rays, and then was led to the invention of the 
eye-mirror, or as he called it, the Ophthalmoscope. 

Instead of placing the light in front of the patient’s 
eye, Helmholtz put it at the side of the patient’s head, 
and reflected the rays by a polished plate of glass into 
the observed eye, while, without great annoyance to him¬ 
self, he looked through the transparent plate into the il¬ 
luminated eye of the patient. Thus did it for the first 
time become possible to observe the details of the interior 
of the eye — its nerve and vessels. All previous observa¬ 
tions on the human eye had been limited to observing sim¬ 
ply its luminosity. Notwithstanding, however, the mag¬ 
nitude of Helmholtz’s discovery, the difficulty of manipu¬ 
lation, the feeble illuminating power, and the limited field 
of view of his ophthalmoscope would in all probability 
have restricted its application to that of a philosophical in¬ 
strument, had not others taken up his idea and, by intro¬ 
ducing great improvements, made it forever the most im¬ 
portant implement to every oculist. Although Helmholtz’s 
instrument was of a crude construction, it does not 
lessen the fame of having opened an inexhaustible mine 
of inquiry; of having shed light on an heretofore chaotic 


THE OPHTHALMOSCOPE. 


143 


darkness, and of having completely revolutionized all 
preconceived notions of the diseases of the deeper 
structures of the eye. It is a striking fact, indeed, that 
the almost unparalleled strides ophthalmic surgery has 
made within late years, date, by a remarkable coincidence, 
with Helmholtz’s immortal discovery. 

The first improvement in the ophthalmoscope was made 
by Theodore Ruete of Leipsic, in 1852, by introducing a 
concave mirror as reflector, which had a small opening in 
the center for the observer’s eye. Since then, mirrors 
of different shapes have completely superseded the plates 
of polished glass. Liebreich introduced a most handy 
and useful instrument. He used a concave metal mirror, 
about li inch in diameter, and of eight inches focal 
length. The back of the central small aperture is bev¬ 
elled off towards the edge, in order that the peripheral 
rays of the cone of light, which passes through it, may 
not be cut off by a thick, broad edge, which would make 
the opening a short tube. Behind the mirror, which is 
fixed upon a short handle, is a small clip for holding a 
convex or concave lens. Other improvements were made 
by Coccius, who introduced a plane mirror, while 
Zehender made use of a convex one, in order to con¬ 
centrate the light upon one point. A further step to 
perfection was made (1870) by E. Gr. Loring, a phy¬ 
sician of New York; his instrument avoids the constant 
changing of lenses behind the mirror, as it contains the 
different convex and concave glasses in three rotating 
cylinders, alternately attached behind the mirror. In 
1873, Dr. H. Knapp did away with Boring’s cylinders 
by presenting an ophthalmoscope with two undetachable 
but revolving discs, one of which containing concave, 
the other convex lenses. These are arranged in such a 
manner that they rotate past each other, so that the focal 
value of each lens can be lessened to a greater or less 
degree by adding to it the various neutralizing lenses of 
the other disc. The glasses are covered by a stationary 
metal plate to prevent soiling. In 1874, Loring simpli¬ 
fied the foregoing instrument, and by adding a few im¬ 
provements produced a comparatively cheap and handy 
ophthalmoscope, which soon took the fancy of most 
oculists. 


144 


HAND-BOOK FOR OPTICIANS. 


All these instruments are monocular; Dr. Griraud- 
Teulon, of Paris, and Dr. J. Z. Laurence, of London, 
invented binocular ophthalmoscopes by combining Helm¬ 
holtz’s invention with Wheatstone’s stereoscope; but 
they soon were dropped, the more handy monocular 
instruments proving preferable. 

The ophthalmoscope finds its greatest usefulness in the 
hands of expert oculists, who have all opportunities of 
learning how to use it. My first experiment with it was 
about twenty years ago. I had an instrument of Lieb- 
reich’s, and my workman and some trusting friends 
were the innocent victims of my investigating proclivity; 
but they soon were frightened by the flashes of light 
which almost blinded them, as they were directed to look 
straight into the mirror, instead of looking a little side¬ 
ways. Since then, there have been published several 
exhaustive treatise on the use of the ophthalmoscope, 
which should be carefully studied before attempting to 
employ it. It is not my intention here to specify the 
many variations and their correction in the observer’s 
and the observed eye, as this treatise is written for 
opticians who, perhaps, never will professionally handle 
an ophthalmoscope. I, therefore, close this chapter with 
some generalities which may give the reader an idea of 
the great importance of Helmholtz’s invention. 

The ophthalmoscope reveals two important conditions 
of the eye; the pathological , by the indirect method, 
where we obtain, by placing a biconvex lens of about 
three inch focus in front of the observed eye, an inverted 
image of the disc ; and the optical, by the direct method, 
not using the convex lens, when we obtain an upright 
image. The indirect method is mostly employed to as¬ 
certain the healthy or impaired condition of the inside 
structures of the eye; the strong convex lens before the 
eye greatly facilitates such an examination. By means 
of this method, experts discover the beginning of certain 
serious diseases, as for instance, Bright’s Disease, before 
any other symptoms of it show themselves. The direct 
method, on the contrary, reveals the optical condition 
of the eye with great certainty, and shows the myopic, 
hypermetropic or astigmatic errors of the eye. We may 


THE OPHTHALMOSCOPE. 


145 


say, therefore, that ophthalmology is an exact science; 
in no other branch of practical medicine or surgery can 
an equally certain diagnosis be made. Some years ago 
I was introduced to one who pretended to know all about 
a person’s general state of health and mind by looking 
at him without asking a single question; but, as ridic¬ 
ulous as were his pretensions, we have to bow to the 
expert oculist who accomplishes this feat by looking into 
the eye with the ophthalmoscope. 

To make a professional examination we first have to 
adjust our own eyes to the focal distance of six or eight 
inches, by means of the convex or concave lenses in the 
revolving discs attached to the ophthalmoscope; then, 
when we place our eye at the given distance, and we find 
the retina sharply focused, the eye observed will be 
emmetropic. But when the image is indistinct, and we 
only can gain a clear view of it by changing our own 
correcting lens to a stronger one, there is hypermetropia 
present in the observed eye, and the difference between 
the two lenses is the amount of its ametropia. Again, 
when we have to reduce the power of our correcting 
lens to a weaker one, then the eye observed is myopic, 
and the difference between the two lenses represents the 
amount of myopia. 

This result is only approximative, as the observed eye 
remains in full possession of its accommodation , which 
greatly interferes with the exact measurement of its 
refractive errors. In many cases it is necessary to sus¬ 
pend the power of accommodation by the use of a 
mydriatic before we can measure the exact amount of 
the refraction , as this is not affected by the application 
of the mydriatic. Some of my readers may not be posted 
about the difference between the accommodation and the 
refraction of the eye; it will, therefore, not be out of 
place to define in a few words their meaning. When 
the crystalline lens is in a state of rest it still has a cer¬ 
tain amount of refractive power, which is, as the mathe¬ 
matician would say, “constant”; but as long as the 
lens is under the control of the ciliary muscle, its power 
of refraction is changed by the alteration of its convexity, 
it is “variable.” This changing of its form or shape is 


146 


HAND-BOOK FOR OPTICIANS. 


called 44 accommodation,’’ and to suspend this 4 ‘variable 
factor ” in order to measure only the 44 constant factor,” 
the refraction , it is necessary to apply the mydriatic. 

As valuable, yea indispensible, as this instrument is 
to the medical faculty, it is of but small importance to 
the dispensing optician. It is not only difficult for us 
to find the many willing patients for experimental pur¬ 
poses in order to acquire expertness, but it is also a very 
unprofitable and time-losing business as long as patients 
refuse to pay for our trouble, in addition to the regular 
price of the spectacles we select for them. 


CHAPTER XIX. 


Second Sight. 


The eye is by no means a perfect optical instrument. 
Its defects are, under ordinary circumstances, suppressed 
by the brighter and more perfectly formed central por¬ 
tion of the retinal image, so that the defects, when not 
of too high a degree, are unobserved and can only be de¬ 
tected by careful experiments. Helmholtz once re¬ 
marked: “ If an optician wanted to sell me an instru¬ 
ment which has all these defects, I should think myself 
quite justified in blaming his carelessness in the strongest 
terms, and give him back his instrument.” And yet, 
nobody can devise a better plan for the construction of 
this organ to accomplish all its duties with such simple 
means. It is the most wonderful machinery ever de¬ 
signed, although it is not perfectly achromatic, and be¬ 
sides, suffers from spherical aberration. Yet, we have 
no reason to grumble at trifling imperfections; we ought 
to accept this precious gift of nature with reverential 
gratitude, and take proper care not to hasten unneces¬ 
sarily its natural decline by an inconsiderate, reckless 
use. But the most careful use of our eyes cannot defer 
the senile changes which take place in all eyes, in myopic 
and hypermetropic as well as in emmetropic eyes, on ac¬ 
count of the natural development of the crystalline lens. 

In childhood the nucleus of the lens is firm, while the 
density diminishes toward its periphery; this arrange¬ 
ment almost entirely overcomes the spherical aberration, 
as the peripheral rays are less refracted than they would 
be, if all parts of the lens were of a uniform density. 
Hence, the circumferential rays are united at nearly the 
same point as the central rays; consequently, the child 
can have a very large pupil, and the peripheral rays still 
be united in the focus of the central ones. With the 




148 


HAND-BOOK FOR OPTICIANS. 


advance of age, the outer layers increase in firmness; 
they gradually approach the consistency of the nucleus. 
The greater firmness and more uniform consistency of 
the lens causes it to become flatter, thus diminishing the 
refractive power of the lens, and increasing its spherical 
aberration. The peripheral rays are brought sooner to 
a focus than the central, thus compelling the pupil to re¬ 
duce in size; the iris acting now as a diaphragm in a tel¬ 
escope. 

The time for the use of spectacles has arrived, and 
should not be overlooked by those who wish to preserve 
their eyesight. The increase in the strength of specta¬ 
cles will keep step with the gradual hardening and con¬ 
sequent flattening of the lens. The development of the 
crystalline can be compared to a fruit while growing; 
it takes its natural course till it is ripe. We cannot, di¬ 
rectly, weaken the lens, no matter how long it is used; 
but, indirectly, it is impaired by the other parts of the 
eye which we abuse or hurt,—as the fruit is prematurely 
ripened by the sting of an insect. When we unduly 
postpone the use of spectacles, we do not weaken the 
lens, but the ciliary muscle, and when we overwork the 
eye, it is not the lens that suffers, but the retina. By an 
inflammation of the cornea or iris, the lens is only sec¬ 
ondarily affected; choroiditis and retinitis have the same 
effect. Each suffering part acts sympathetically upon 
the others by reflex action. 

One of the most dreaded affliction of the eye is when 
the lens commences to lose its transparency, when signs 
of cataract make their appearance, and people anticipate 
blindness and misery. Cataract is, generally, a disease 
of old age; the loss of the transparency of the lens is 
chiefly due to its deficient nutrition, dependent upon an 
inefficient blood supply, and consequent diminution of 
the watery constituents of the crystalline lens. Inflam¬ 
mation of the inner tunics of the eye, especially of the 
iris, choroid, and vitreous humor, may also give rise to 
cataract, not by an impairment of the nutrition of the 
lens, but by the inflammatory changes, implicating the 
inner capsule, and even the lens itself. — It is very 
difficult to detect the real cause of cataract. Anions: the 


SECOND SIGHT. 


149 


most important of these causes is exposure to light and 
heat; for instance, the artisan at his work-bench, facing 
with his unprotected eyes a window or gas-jet for many 
hours every day; the cook, bending over the heated 
range ; glass-blowers, bakers, blacksmiths, puddlers, 
stokers and engineers are affected. A fully formed, 
mature cataract may be easily recognized even with the 
naked eye; the pupil is no longer dark and clear, but is 
occupied by a whitish opalescent body, which lies close 
behind it. However, when cataract is incipient , and 
but slightly advanced, more especially when the opacity 
commences at the edge of the lens, it may be overlooked, 
except when the eye is carefully examined with the 
ophthalmoscope. 

Care must be taken not to mistake the physiological 
changes which occur in the lens in old age for commenc¬ 
ing cataract. These changes consist in a thickening 
and consolidation of the lens substance, especially of the 
nucleus, which assumes a yellowish tint. The chief 
distinctive features between this and incipient cataract 
are, that in the former case the sight is perfect, when 
assisted by suitable glasses; the opacity remains abso¬ 
lutely or almost entirely stationary for a long period , 
and the cloudiness is not observable with the ophthal¬ 
moscope, except with oblique illumination. We have 
•here a clear case of second sight. 

Since the issue of my first book, I made diligent inquiry 
of several cases, and learned that their development is 
not of such short duration as I imagined. A former 
customer laid aside her spectacles at the age of 66 years, 
and is now, after 17 years, still doing fine needle-work 
as well as ever. Of course, her distant vision is poor, 
and ought to be corrected by concave glasses; but she 
hates to commence again with spectacles. Other cases, 
of a shorter standing, known to me, show that second 
sight is not always a gift of Danaus,* which ends in 
misery, but that it is sometimes a blessing, and a source 
of great rejoicing for those people who perceive that one 

* Danaus was the king of the most prominent province in ancient 
Greece. It is told that his gifts were often disastrous to the receiver. A 
gift, therefore, given with bad intention, is called after him. 



150 


HAND-BOOK FOR OPTICIANS. 


faculty after another gradually withers and vanishes, 
except their eyesight. 

In the development of real cataract we meet with a phe¬ 
nomenon which may falsely be taken for second sight, 
and cruelly disappoints the afflicted, as it is, generally, 
of short standing. Dr. Soelberg Wells says: “The rate 
of progress of senile cataract is very difficult to determine 
with accuracy. Sometimes, years may elapse before it 
arrives at maturity. It may remain at an incipient stage 
for a long time without apparently making any progress, 
and then suddenly advance very rapidly, arriving at 
maturity within a few months or even weeks. We must, 
therefore, always be upon our guard against giving a 
decided opinion as to when any given case of incipient 
cataract will be fully formed. Patients are sure to ask 
this question, and we may fall into great mistakes by 
giving a decided answer.” Another physician recently 
remarked: “When oculists, formerly, were consulted 
for relief from commencing cataract, it was their habit to 
acquaint the patient or his friends with the cause of the 
failing vision. The opinion expressed was to the effect, 
that for the present nothing could be done to restore vision; 
on the contrary, it would grow steadily worse, but that 
though blindness might and probably would ensue in one 
or both eyes, vision could be restored by removal of the 
ripe cataract. Thus the patient and the family were 
sent away with an abiding solicitude hanging like a cloud 
over the household, the anxiety alleviated only by the 
prospect of a future successful operation.” 

Such sensible remarks teach the wholesome lesson to 
opticians, who have to deal with those customers, seeing 
the advance of this fearful visitation, not to be indiscreet, 
and wantonly dispel their happy delusion, as nothing in 
the world can arrest the final course of their trouble. 
We may advise them not to read or sew at night, and to 
spare their eyes as much as possible. When in the first 
stage of incipient cataract bright light begins to annoy 
their sight, give them smoked glasses; they neutralize 
the scattered rays which pass through the infected lens. 
Do not lose patience by their renewed attempts to find 
relief by changing their spectacles. Have always a kind 


SECOND SIGHT. 


151 


word for them, and as you cannot help them materially, 
let them have the full benefit of your benevolent sym¬ 
pathy. 


CHAPTER XX. 


Relief to Injured Eyes. 


This chapter is partly compiled from different sources. 
It would not have found a place here if it were not for 
the great usefulness of these simple directions in case of 
emergency. 

Though the eyes are well protected and shielded by 
the forehead, the nose-bridge and the cheek-bone^, they 
are nevertheless exposed to accidents caused by small fly¬ 
ing objects; and although the eyelids are reliable safe¬ 
guards to keep off any foreign intruders, they may be 
out-generaled occasionally when they are the least aware 
of any danger. Some injuries do not allow of any delay, 
and as medical assistance is not always to be had when 
mostly needed, I thought it proper to add this treatise 
not only for the personal benefit of my readers, but also 
for that of their friends and customers, who may in their 
trouble come running to the optician to give them relief. 
I was several times successful in this respect, and may 
say that I saved more than one eye from great annoyance 
and danger. 

A very common accident is the flying of mud , dust or 
insects into the eye, which, by the closing of the eye, en¬ 
ter between the lid and the eye-ball. People thus af¬ 
fected generally keep their eyes closed, as the opening 
of the lids causes such an irritation that the eye-ball is 
soon inflamed and bloodshot. The quickest way to re¬ 
lieve these sufferers is to wash the dirt out with clean water 
by means of a camel-hair brush or a feather. This is done 
in the following manner: With our left hand we take 
hold of the eye-lashes of the upper lid, drawing it for¬ 
ward sufficiently to allow the brush or feather, previously 
dipped in water, to enter between the eye-ball and lid, 
till we reach the inner folds. We direct the patient to 




RELIEF TO INJURED EYES. 


153 


look downward, and move the brush towards the nose, 
not to the outside. We have to repeat this several times 
with plenty of water. Then we depress the lower lid, 
directing the patient to look upward, and wash carefully 
as before, cleaning the brush after each application. In 
some trifling cases, when an insect or a few grains of dust 
have entered the eye, draw the upper lid as far down as 
possible, a little outward, and push the lower one as far 
up as you can. Then let the upper lid fly back to its 
natural position, when the eye-lashes of the lower one 
will act as a brush, detaching any light substances, and 
relieving the eye instantly. Make it a rule never to rub 
the eye when injured, as the irritation will be increased 
largely by it, and soon will cause inflammation. When 
hard pieces are imbedded in the tender parts of the con¬ 
junctiva, which cannot be removed by the brush, it is not 
difficult to remove them, if they are lodged in the lower 
lid, by means of a handkerchief or some small pincers; 
but it requires some skill to remove them from behind the 
upper lid. In order to accomplish this, we have to 
evert the same, which is done by taking a good hold of 
the eye-lashes and the edge of the lid with the left hand, 
then applying with the right hand a thin pencil or any 
other rounded object to the middle of the lid, and by de¬ 
pressing the pencil, at the same time swinging the 
left hand upward, the lid is everted and the inside ex¬ 
posed for examination. The patient is now directed to 
look downward, which brings into view the whole inner 
surface of the upper lid, and enables us to remove any 
foreign bodies, as grains of sand or bits of coal, yet stick¬ 
ing in the soft part of the tender tissues. 

A somewhat singular advice, how to remove grit from 
the eye, was lately communicated by a railroad man. He 
says: 4 4 Most persons with grit or any foreign substance 

in the eye will instantly begin to rub the organ with one 
hand, while hunting for their handkerchief with the other. 
They may, and sometimes do, remove the offending sub¬ 
stance; but more frequently they rub until the eye be¬ 
comes inflamed, then bind a handkerchief around the 
head and go to bed. This is all wrong. The better way 
is not to rub the eye with grit in at all, but rub the oilier 


154 


HAND-BOOK FOR OPTICIANS. 


eye as vigorously as possible, causing the offended eye to 
profusely shed tears by which the bit of sand or dust is 
washed out.” 

Mechanics are very often hurt by flying particles of 
metal while hammering or turning, and chips may strike 
and penetrate to some extent the front part of the eye. 
If these are of iron or steel, and not imbedded too deep, 
we may remove them by the use of a strong magnet. In 
case these chips have penetrated so deeply that the con¬ 
junctiva has closed over the entrance of the wound, it is 
necessary to consult a physician. Such wounds are not 
very painful at first, and the application of water or oil 
may be sufficient to allow us to wait even until the next 
day to look for relief. Any longer delay may prove 
fatal, as a neglect will surely result in a violent inflam¬ 
mation, if these particles are not removed in due time. 

Another danger to the eyes is the splashing of quick¬ 
lime into them, causing sometimes the complete loss of 
sight. I myself was a victim of such an accident at the 
age of four years. Some workmen w r ere slacking lime, 
and I was wondering how stones covered with water 
could boil. Wholly absorbed by this phenomenon, one 
mischievous boy gave me a push, and I fell headlong 
into the hot lime water, but was immediately rescued, 
washed and brought to bed. I soon felt that something 
soothing was applied to my eyes, which relieved them of 
the burning sensation. It was three weeks before I 
could open my eyes again, and I remember quite well the 
many anxious inquiries of my parents, whether I could 
see them. In such accidents, the lime should be instantly 
washed out with large quantities of weak vinegar and 
water as thoroughly as possible, and a rag saturated with 
sweet-oil applied, till a physician can be consulted. 

If corrosive pigments and acids enter the eye the whole 
face, eyes open, should be repeatedly dipped in water in 
order to dilute and wash off the acid or paint ; then apply 
milk freely, and afterward plenty of oil, till medical as¬ 
sistance can be procured. Whatever is done must be 
done quickly, as it is of the greatest importance to re¬ 
lieve the eye instantly from the ravages of such corrosive 
substances. 


RELIEF TO INJURED EYES. 


155 


In case the eye should be scalded or injured by the 
spattering of hot fluids, do not apply water, but only oil 
or milk, and shut off light and air by a compress of soft 
linen, thoroughly saturated with sweet-oil, till the doctor 
comes. — Hot vapors of strong sulphuric or nitric acids 
will cause immediate blindness if they strike the eye. 

These directions are not intended to do away with the 
services of the physician. On the contrary, they are in¬ 
tended only to prevent as much as possible the pernicious 
consequences and further progress of such accidents, till 
professional aid can be procured. Sometimes five min¬ 
utes’ delay may destroy the eyesight forever, when by 
the prompt application of water, vinegar, milk or oil, the 
effects of such injuries would be diminished, and often¬ 
times removed entirely. 


11 


CHAPTER XXI. 


Abtificial Human Eyes. 


Long before the Christian era, artificial eyes were 
used in statues and busts; they were made of colored 
stones or metal, and inserted into the cavity expressly 
constructed for their reception. The deep sockets found 
in some statues without eyes are an evidence of this; 
like that of Antinous in Paris, and others in different 
archseologic museums. In some ancient statues the eyes 
were outlined by the chisel and then painted. We have 
no record that glass-eyes were ever used for this purpose, 
although colored glass was already known at that period, 
and the story that mummies with artificial eyes were 
exhumed is probably without foundation. It was abso¬ 
lutely unnecessary to ornament a body which was enveloped 
all over, several inches thick with bandages, and then 
covered hermetically with a kind of plaster or cement. 
This outside coating was made to represent a human-like 
figure, studded with glass beads and other ornaments; 
but no mummies have yet been found with artificial 
eyes, except those lately discovered in South America, 
which are of modern date, only four or five hundred 
years old, of the time of the ancient Incas of Peru. 
The custom of embalming was very common among the 
Incas, and was made unusually easy by the warm, dry 
climate of Peru. It is stated that the embalmed were 
often simply placed in a sitting posture on the vast nitre 
beds, and left exposed to the open air. For years after 
death they were visited by friends and relatives, and 
it was consequently important that the semblance of life 
should be maintained as perfectly as possible. They 
removed, therefore, the perishable natural eyeballs of 
the dead, and substituted the dried eyes of the cuttle 
fish, which are almost indestructible, and possess suffi¬ 
cient warmth and fire to partially simulate life. 




ARTIFICIAL HUMAN EYES. 


157 


The substitute for a live human eye is not older than 
three hundred years. We find the first authentic record 
of them in the Chirurgical Work of Porree, 1582, with 
drawings of two kinds of eyes, one to represent the eye 
and the lids, in case both were removed, the other to be 
used when the lids were still present, and to be inserted 
behind them. Both kinds were made of flat or slightly 
curved gold, silver or copper plates, enameled and painted 
to imitate the other eye as near as possible. When the 
eyeball and lids were totally removed by the operator, as 
was done very often at that time, the plate had to be 
large enough to represent also the eye-lids, lashes and 
caruncle. To this plate was attached a spring covered 
with leather, which encircled three quarters of the head, 
thus pressing the plate towards the hollow orbit. This 
was indeed a very poor commencement of that great 
benefaction of to-day. Porcelain, and afterwards glass, 
soon took the place of metal, asFabricius states, in 1623. 
From that time the manufacture of eyes has slowly but 
constantly improved. Dr. Mauchard, 1749, relates of a 
lady who had been furnished by him with an artificial 
eye, that she only wondered why she could not see with 
such a beautiful imitation. 

Since 1770, the eyes have been made of enamel in¬ 
stead of glass, mostly in France, and for many years, 
up to recent date, the best eyes have come from Paris. 
Some of the celebrated manufacturers were Hazard- 
Micault, Noel, Boissoneau, etc. But at present good 
eves are made everywhere, even in America. The best 
eyes I ever handled were made by Ludwig Muller-Uri, 
in Germany, who lately died. 

An artificial eye is a shell of enamel, representing the 
front of the eyeball, the loss of which it is intended to 
conceal. They are so perfected that nobody can distin¬ 
guish them from the natural eye, and they are now used 
by everybody who possibly can afford to pay for them. 
An artificial eye is by no means a luxury as in former 
years, when only rich people could afford to pay its 
price, but it is now altogether a necessity for everyone who 
is unfortunately disfigured by the loss of it. The sev¬ 
eral functions it has to perform, are: 


158 


HAND-BOOK FOR OPTICIANS. 


1. The artificial eye serves as a heautifier. It restores 
the natural appearance of the face and preserves the 
regularity of the features. If, for instance, the greater 
part of the eye-ball is lost, the lids have no support and, 
consequently, shrink and shorten. When this loss 
happens in the earlier stage of life, the development 
of that side of the face becomes retarded and will present 
a strange appearance. Even in adults this shrinkage 
takes place after the lapse of a few years. It is, there¬ 
fore, wrong to postpone the use of an artificial eye, 
especially with children whose tissues change so rapidly, 
the more so, as even a child of five years can wear an 
artificial eye without any inconvenience. 

2. The artificial eye serves as a, remedy. It enables 
the eyelids to move freely, they can be closed and opened 
as before, and also restores the functions of the tear- 
passages. After the removal of the eyeball there is an 
empty space left behind the lids; the tears accumulate 
in this cavity, and irritate the edge of the lower lid, and 
frequently give rise to ulceration. By means of an arti¬ 
ficial eye, however, the tears are directed to their natural 
channel, and are removed in the usual way through the 
nose. It also prevents the eyelashes from turning inwards, 
causing inflammation and suppuration by their constant 
friction upon the structures left behind in the socket. It 
protects the latter from all outside irritation, such as 
wind, dust, smoke, etc., which otherwise might sympa¬ 
thetically exert a pernicious influence upon the remain¬ 
ing sound eye. 

3. The artificial eye is a real benefaction . It is a 
blessing to everybody, for the rich as well as for the 
poor; but while it serves the former mostly as a beauti- 
fier, it very often protects the latter against want and 
misery. It is really a question of existence for them, as 
some employers will hesitate to engage a person thus 
disfigured; or trust him with work that requires good 
sight, even if it could be well performed with only one 
eye. By the use of an artificial eye, however, such 
objections are removed and he may readily find employ¬ 
ment. 




ARTIFICIAL HUMAN EYES. 159 

A judicious selection of the different sizes of artificial 
eyes depends altogether upon the various dimensions of 
the cavities. Children, generally, need larger eyes than 
elderly persons, and a great variety as to shape and color 
is, therefore, necessary to suit all cases. The white of 
most artificial eyes is on one edge more or less cut out, and 
as a rule, this edge should be used for the upper lid, in 
order to allow the same as much room and liberty of mo¬ 
tion as possible. We may classify the eyes, therefore, 
into right and left ones, according to the position of this 
cut-out. By doing this, we must bear in mind that the 
smaller end of the eye is the nasal part. But when the 
artificial eye squints upward, we have to change this rule, 
as the true position of the pupil and iris is the principal 
condition of a good fit. The eye should never cover the 
caruncle (the fleshy protuberance in the inner canthus, or 
in the corner of the eyelids near the nose), and should 
allow the lids to close. 

In order to make the artificial eye light, and save the 
under lid from being depressed by its weight, it is made 
in form of a shell with its border finished oft by the melt¬ 
ing process with a pointed flame. Any alteration by cut¬ 
ting and polishing it, will render the eye useless after a 
short while. 

Before the insertion of an artificial eye, the tissues in 
the socket must be perfectly healed and cicatrized, and 
the conjunctiva free from inflammation and morbid sen¬ 
sibility. Ail artificial eye, besides resembling the oppo¬ 
site sound eye in prominence and in color and appearance 
of the iris, ought, if the stump be good, to move in con¬ 
cert with it. This it does by following the motions 
communicated to the conjunctival folds, into which its 
margins are fitted, and by the movements of the stump. 
It ought at the same time to cause no pain or uneasiness. 
A perfect motion of the eye is possible only when the 
stump is large, and the eye is almost resting on it. For 
this purpose the eye must be rather flat, not.too large, 
lest the free motion will be checked by its touching the 
sides of the orbit. When the stump is small, the artificial 
eye must be large, high and well rounded. The inner- 
folds of the eyelids are then its only support, and its 


160 


HAND-BOOK FOE OPTICIANS. 


motion is, of course, little and insufficient; the more so, 
if the patient insists on having the artificial eye matched 
exactly in size with the other, thus producing a “ staring 
look.” 

To introduce an artificial eye, it is necessary to raise 
the upper eyelid and slide the eye, previously dipped in 
water, up behind it by the end which is to correspond 
to the temporal angle. Then turning the nasal part up¬ 
ward, and letting the upper eyelid fall, depress the lower 
forcibly, and make the lower edge of the artificial eye 
slip into the lower palpebral cavity. This being done, 
and the lower lid allowed to rise, the introduction of the 
eye is accomplished. 

The removal of an artificial eye is done in the follow¬ 
ing manner. It is withdrawn by an opposite procedure, 
by depressing the lower lid, and inserting the curved end 
of a hair-pin, or even the thumb-nail, between the eyelid 
and the lower edge of the artificial eye, thus hooking it 
out from the lower palpebral cavity, when it will glide 
down from behind the upper eyelid, and fall into the 
hand ready to receive it. In doing this himself, the 
patient should lean his face over a soft cushion, or the 
like, in order that, if the eye should slip out of his fingers, 
it may not be broken in the fall. 

The artificial eye is withdrawn every night, and is to 
be cleaned with w 7 ater (which should be tepid in winter) 
of the mucus which may adhere to it. Before putting 
in the artificial eye and after withdrawing it, the patient 
should bathe the cavity or the orbit and the stump of 
his eye with water. A thorough cleaning of the artificial 
eye every week with soap, water and a soft sponge is 
also recommended in order to remove all fatty matter 
from it. But altogether objectionable the habit of 
some people to oil their artificial eye before inserting it, 
because the mucous membranes of the orbit do not re¬ 
quire such additional lubrication, as long as the eye has 
not lost its polish.—In the course of time it becomes rough 
from the slow corrosive action of the humors which 
come in contact with it, and requires to be exchanged 
for a new one. In case there should be some difficulty 
in procuring a new eye, the old one may be repolished 


ARTIFICIAL HUMAN EYES. 


161 


and do service for a few months longer. You take a 
rag of cotton, form it into a small ball and fasten the eye 
over it with soft bee’s wax. The inside of the eye should 
be well filled up to prevent any accident. Then put in 
your hand a little alcohol or water and fine pulverized 
English rouge, and afterward Parisian rouge to finish, 
and shine the eye as you would shine a brass button. 

A worn-out eye causes congestion of the lids, a swell¬ 
ing of the conjunctiva and a gradual filling up of the 
orbit. Many unfortunate sufferers have in this way 
deprived themselves entirely of the great blessing of 
correcting their disfiguring loss by a suitable substitute. 
As an artificial eye is liable to be broken by accident, 
a person making use of it should always have several on 
hand. An eye will last no longer than two years on an 
average. From the irritation excited by the artificial eye, 
when it is either a bad fit or worn too long, the palpebral 
conjunctiva are apt to become much congested, and 
beset with polypus-like excrescences. In this case the 
use of the artificial eye should be discontinued for some 
time, and it is necessary for the patient to seek medical 
assistance. 


CHAPTER XXH. 


Caloric Rays in Different Lights. 


According to the old emission theory, light is a com¬ 
pound matter ; but accor din g to the new undulatory 
theory, it is a compound force. It is a mixture of lu¬ 
minous and caloric wayes, and is also a combination of 
the different colors. To resolve light into its colors has 
been a comparatively easy task since the properties of 
the prism have been known; but the complete separation 
of luminous from the caloric rays is yet a matter of inves- 
tioration. Eminent scientists haye labored long to isolate 
one from the other, but only with partial success. 
Light, passing through an ice block, or through plates of 
mica, is not deprived of its caloric rays, although they 
are absorbed to a certain extent by reflection; but by 
means of a strong burning-glass we detect enough of 
them to be aware of their presence. Some explorers 
have succeeded in completely absorbing the luminous 
rays, and showing the presence of only the caloric rays 
in their full strength. 

The folio win 2 experiment was communicated to me by 
Professor Pepper, of England, in 1872, when he, on his 
lecture tour, passed through New Orleans. I repeat it 
here as he explained it to me. I have never tried this 
experiment myself. I remember with great pleasure his 
able lecture on 64 Light and Heat,” illustrated profusely 
by novel and highly interesting experiments. 

The candle 5 stands between the glass-jar c and the 
concave mirror a. The rays of the candle are thrown 
by the mirror on the flat jar, filled with a solution of sul- 
phuret of carbon and iodine, which completely absorbs 
the luminous rays. You cannot detect through the jar 
the least trace of light; but if you hold your finger at the 
point d you will find that the caloric part of the light is 


CALORIC RAYS IN DIFFERENT LIGHTS. 


163 


concentrated there most keenly. This shows that the liquid 
absorbed only the luminous rays, and allowed the caloric 
rays to pass through without perceptible interference. 



In the same manner that luminous rays are modified or 
intercepted while passing through bodies of different de¬ 
grees of clearness, the caloric rays are also more or less 
intercepted by different substances. Mica, for instance, 
absorbs the greater part of the caloric rays; but the only 
substance which allows all caloric rays to pass without 
any obstruction is clear rock-salt. Experiments with 
prisms of this salt have demonstrated the fact that light 
passing through such a prism gives two spectra, one by 
the luminous, another by the caloric part of the light, 
with the remarkable difference that the red line of the 
caloric spectrum is as broad as all the other colors com¬ 
bined, from orange to violet. This experiment is an un¬ 
deniable proof that the caloric and luminous rays can be 
separated, and that both kinds of rays are subject to the 
same law of nature, the undulatory or wave theory. 

This theory defines light as motion of such an intense 
velocity that we can express it only in figures, but are 
utterly unable to comprehend it. Imagine that we were 
able to build a machine of indestructible material, and 
had the power of increasing its revolutions indefinitely. 










164 


HAND-BOOK FOR OPTICIANS. 


We put it into operation. As long as we can follow its 
movements with the eye, we have common motion. We 
can follow with our eyes a stone thrown to some dis¬ 
tance. This also is common motion. Let us now in¬ 
crease the speed of our machine thirty-three revolutions 
a second. The eye can no longer follow it, hut the ear 
discerns a low hum, which becomes louder and higher as 
the machine gradually moves quicker. We have sound . 
A rifle-ball is not seen, but we hear its whistling noise. 
When the tone has reached its highest pitch (38,000 vi¬ 
brations in a second), our ear is unable to perceive any 
further increase; we feel then the effect of heat , and soon 
see a violet glimmer, then a transition through blue, yel¬ 
low and red into white. We now have light. The vi¬ 
brations have increased to many thousand billions a sec¬ 
ond. If our machine is not melted by this time, and is 
still running with increasing speed, we had better keep at 
a safe distance, for the next action will be the emission 
of electric sparks and lightning in all directions. 

Here science ends, and here is the limit of all power 
and force we can explain or comprehend. But if we al¬ 
low our imagination its widest range, and look upon this 
experiment only as the symbol of the universal, sublime 
power, does it not give us a faint idea of the proper 
mode of attaining to the knowledge of the ultima ratio , 
the incomprehensible omnipotence? 

Light and heat are always combined; there is no light 
without heat, for phosphorescence cannot be regarded as 
light. Of all the lights in existence, the natural or sun¬ 
light is the most pleasant; it has 70% of caloric and 30% 
of luminous rays. The great preponderance of the ca¬ 
loric over the luminous rays is necessary to make our 
earth habitable, as its natural heat, about one hundred 
feet below the surface and not interfered with by atmos¬ 
pheric changes, is only 50° Fahrenheit. Although the 
temperature increases 1° for every 65 feet of depth, so 
that at two miles below the surface water will boil, and 
at thirty-four miles, iron will melt; the inner heat, esti¬ 
mated at more than 10,000°, is not able to warm the com¬ 
paratively thin crust of our earth much above the freez- 


CALORIC RAYS IN DIFFERENT LIGHTS. 165 

ing point. Besides, if the sun would not come to our as- 
sistance, we could not endure the low temperature of 
the Universe, which is calculated to be 2000° below 
zero. But the sun with her 100,000° of heat on her sur¬ 
face, overcomes all these obstacles, and sends us sufficient 
light and heat to make our earth the most pleasant quar¬ 
ters to live in. 

We must not form the wrong idea that the immense 
radiating heat of the sun could extend through the whole 
solar system and reach the last planet, Neptune; in fact, 
it does not reach even the nearest planet, Mercury.* — 
The real size of the sun can be best demonstrated by sup¬ 
posing the sun to be a hollow shell, with our earth in its 
center and the moon moving around the earth at the same 
distance. Now, if we imagine ourselves to be present on 
the surface of the shell, and looking at the moon through 
a hole, it would in its nearest position to us appear only 
of the same size as when viewed from the earth. Just 
think, the sun to be a solid body with a diameter of four 
times the distance of the moon from the earth, and you 
have an idea of its enormous size. And, further, imagine 
this immense ball to be in a state of combustion 1 Not 
calmly glowing as it appears to us from a distance of 95 
million miles, but in a state of furious uproar and thun¬ 
dering convulsions. Just look at a large house on fire, 
and notice the crackling and hissing of the flames; watch 
with awe the fearful roaring and thundering of a burning 
city; picture to yourself, if you can, the terrific reports 
and unearthly glare when a stream of lava bursts through 
the sides of a volcano; the vast flames leaping hundreds 
of feet into the air, amidst the fearful internal rum¬ 
blings; multiply all these a million times, and we may 
get a faint fdea of the sun’s present condition. The ter¬ 
rible roaring would be heard-millions of miles away; tre¬ 
mendous sheets of flames, called protuberances, are 
thrown hundred thousand miles into space; the con¬ 
stant explosions, tearing holes in the surface of the 
sun, causing the sun-spots, which our earth would not 
fill: really, a battle of elements of the sublimest 
grandeur. 

* We know too little of the planet, Vulcano, lately discovered, to use 
it here as an illustration. 



166 


HAND-BOOK FOR OPTICIANS. 


The atmosphere of the sun is calculated to extend 
about five million miles, but its radiating heat will not 
reach thus far, so that the planet, Mercury, whose mean 
distance from the sun is 37 million miles, must have at 
least thirty million miles of that extreme low temperature 
of the universe, and our earth fully 90 million miles of 
it. It is, therefore, impossible that the rays of sunlight 
actually carry particles of its radiating heat with them. 
In fact, the vibrations of ether caused by the sun are by 
themselves neither light nor heat, until they are decom¬ 
posed under certain conditions, as in passing through our 
atmosphere; but how this is performed I must leave to 
professional scientists to explain. 

The sunlight is perfectly white or colorless, and is the 
most agreeable to the eye. The caloric part of it is 
greatly modified by the moist atmosphere it has to pene¬ 
trate, and by repeated reflections. The healthy eye is 
well able to bear its effect the whole day long without 
fatigue. 

Next to sunlight, electric light is the strongest; it has 
a bluish-violet tinge, and contains 80% of caloric and 
20% of luminous rays. The electric light is not pro¬ 
duced by combustion, as we have seen in sunlight, but 
by the intense heating and volatilization of ponderable 
matter, because the electric spark cannot pass through a 
vacuum. It is very intense, so that the eye is dazzled, 
and vision becomes much more indistinct than with a 
light of the same power given off from a lamp with a 
large circular wick.— The first experiments with large 
Voltaic piles to produce an electric spark, were made 
merely for curiosity’s sake, till in 1813, Sir Humphrey 
Davy (1778—1829), a celebrated English chemist, at¬ 
tached to the wires of the different poles, pencils of 
charcoal, and produced a constant arc-light of consider¬ 
able strength. But his experiment was not followed up 
as arduously at the time as it has been during the last 
twenty-five years. The scientists experimented with the 
Drummond or calcium light, and later with Bunsen’s 
magnesium light, without following up Davy’s temporary 
success. In fact, it was merely an experiment and of 


CALORIC RAYS IN DIFFERENT LIGHTS. 167 

no practical use, as the charcoal points were too rapidly 
consumed. A great improvement was made in 1843, by 
Foucault, who substituted pencils of hard gas-carbon, 
such as is deposited in the interior of the retorts during 
the manufacture of illuminating gas. Since then, many 
different forms of electric lamps have been devised. 
They are divided into two kinds: the arc-lamp, for street 
illumination, and the incandescent , for use in houses, 
offices and theatres. The latter is an invention by 
Thomas A. Edison. This light is preferable to gaslight 
on account of its cleanliness and its comparative cool¬ 
ness; it does not fill the room with impure vapors, is 
steady and pleasant to the eye. 

An important improvement in the line of illumination 
was, at the time, the invention and introduction of gas¬ 
light ; it has only 90% of caloric rays, has a yellowish 
tinge, and is often flickering and unsteady and, there¬ 
fore, tiring the eye more than an improved oil-lamp. 
Gas is made from different combustible materials, of 
which the stone-coal is mostly employed. Just one hun¬ 
dred years ago, 1792, Murdoch made the first experi¬ 
ments with it; and already in 1811 some stores and 
streets in London were illuminated with gas. Simul¬ 
taneously Lampadius experimented in Germany, and 
Lebon in France, but with little result; the English were 
far ahead in the manufacture of gas, mostly due to their 
superior coal. Especially after they engaged the ser¬ 
vices of the German chemist, A. Winzer ( Winsor), the 
gas-industry made rapid progress. He collected the gas 
in large reservoirs or tanks, and thence forced it through 
small gas-pipes to the different places of consumption. 
He started many gas-companies in England as well as in 
France. He died 1830 in Paris. Between 1830 and 
1840, almost all large cities had their gas-works, even in 
America. 

Another useful light is the ordinary oil-lamp, with its 
87% of caloric and 13% of luminous rays. The best 
form of it is an imitation of the German student’s lamp, 
with a suitable shade; its light is preferable to all other 


168 HAND-BOOK FOB OPTICIANS. 

artificial lights. — Lamps were known in very ancient 
times, and are already mentioned in Gen. 15, 17, and Ex. 
27, 20. The lamps of the Jews, Greeks and Romans, 
were of a primitive construction; a hollow open vessel 
for the reception of oil, ending in a spout to carry a 
coarse wick, was the whole arrangement, though the ex¬ 
terior was often artistically sculptured. 

The real improvement of lamps began only in 1550, 
when Hieronymus Cardanus constructed a lamp with a 
separate receptacle for oil, which was attached to the side 
of the lamp, and which produced a comparatively steady 
light. Soon the addition of a lamp-shade followed. 
The next important improvement was made by the 
Frenchman, Leger, who invented, 1765, the flat wick , 
which was in 1782 improved into the circular form by 
Argand, who also added the glass chimney to the lamp. 
— The great trouble wdth all lamps was, that a bright 
light needed more oil than the wick was able to conduct 
to the flame, thus causing the wick to coal and compel 
frequent clipping. For this reason, Carcel (1800) com¬ 
bined a clockwork with the lamp to feed the flame suf¬ 
ficiently with oil. In 1809, the so-called “ Astral-lamp ” 
was introduced, which received its name from the round 
oil-receptacle placed below the flame; all lamps, hereto¬ 
fore, had their oil-vessels either sideways or above the 
flame. 

A complete change in the construction of lamps was 
caused by the introduction of petroleum, or coal oil, as a 
light producing agent, which generates combustible va¬ 
pors at a much lower temperature, and being a thinner fluid 
than other oils in use, it moistens the wick quicker and 
follows it to a greater height. But such a flame requires 
a good ventilation, which serves also to cool the burner; 
at the same time, the oil-fount has to be placed far be¬ 
neath the flame to prevent the heating of the oil. When 
the wick is flat, the burner is in need of a semi-spherical 
metal cap with a slit, a little larger than the opening in 
tjae wick-guide, to allow a free passage of the flame, and 
where its vapors unite with the oxygen of the air, which 
favors a better combustion and prevents the flame from 
smoking. When the wick is circular, the metal cap is 


CALORIC RAYS IN DIFFERENT LIGHTS. 


169 


replaced by a chimney, where the wider lower part is 
suddenly reduced into a smaller cylinder, while the chim¬ 
ney of the flat wick is bellied. 

Another source of light is the candle , which has nearly 
the same proportion of caloric and luminous rays as the 
lamp. Candles were not known before the second cen¬ 
tury after Christ, although lamps were used for over two 
thousand years, before anybody conceived the idea that 
solid fatty matter, like tallow and wax, would answer the 
same purpose. Candles have the advantage that they do 
not smoke as much as the oil-lamps, but are more expen¬ 
sive, especially the wax-candles. For many hundred 
years they were used only in churches on certain solemn 
occasions, and by the rich and reigning households as a 
sign of luxury. — About the year 1700, the spermaceti- 
candles were introduced; their light was extremely white, 
but the price was considerably higher than even wax-can¬ 
dles. The spermaceti is a fatty substance from the head 
of the cachelot ( potfish or white whale); sometimes one 
single fish produces twelve barrels. Such candles were 
mostly used to compare and measure the intensity of 
different lights, but were too costly for every day use. 
Since 1725, the cheaper stearine-candles have been great 
rivals of the still crude lamps of that time, especially 
since the invention of braiding the wick, although the 
improved lamps gradually superseded the candles, ac¬ 
complished finally by the introduction of coal-oil. In 1831, 
De Miliy invented a simple and cheap method of pro¬ 
ducing stearine; the manufacture of candles was again 
greatly revived, but their most successful rival is now 
the gas. The poorest light of all is the alcohol-lamp, 
which has only £% of luminous rays, and is absolutely 
unfit for seeing-purposes. 

It now remains to draw attention to the action of the 
different lights upon the eye, and to show the impor¬ 
tance of knowing the exact proportion of the luminous 
and caloric rays in either of them. We have seen in a 
previous chapter that the size of the pupil is governed 
by the action of the iris, and as the iris is affected only 


170 


HAND-BOOK FOR OPTICIANS. 


by luminous rays, it is evident that light which contains 
the largest proportion of them will contract the pupil 
more than another light with less luminous rays. The 
30% luminous rays of the sunlight will, therefore, 
contract the pupil most, and will allow but a limited 
amount of caloric rays to enter the eye. Any light with 
a less proportion of luminous rays causes the pupil to di¬ 
late, and favors the entrance of a greater amount of ca¬ 
loric rays without improving sight. Therefore, the large 
quantity of caloric rays in all artificial lights will sooner 
fatigue the eye than the comparatively cool sunlight. If 
we visit, for instance, a well-lighted theatre during a 
“ matinee,’’and there are exposed for hours to the daz¬ 
zling gaslight, we feel greatly relieved when sunlight 
again strikes our eye. 


CHAPTER XXIII. 


Range of Vision. 


In Genesis, chap. 11, 1—9, we find the amusing story 
of the building of the Tower of Babel, “ whose top may 
reach unto heaven, and may be seen upon the face of the 
whole earth.” People were afraid to stray away too far 
from the center of their colony, partly for fear of being 
lost in this wide world, but mainly for fear of tumbling 
down, somewhere, over the edge of the flat circle, which 
the ancient believed the true shape of the earth. 

The first one who showed practically that the earth is 
a globe, was Christopher Columbus (1435—1506)*, who 
maintained that by sailing westward, one would reach 
the East-Indies sooner than by the south-east course 
round Africa. He explained his plan, after many pre¬ 
vious failures in Italy and Portugal, to Ferdinand and 
Isabella of Spain; but only after an eight years’ struggle 
with the obstacles thrown in his way by ignorance and 
malice, especially by the fanatical priests of the Spanish 
Inquisition, he received three small vessels with 120 
men. Eighteen years had elapsed since he first con¬ 
ceived the idea of his enterprise. Most of that time 

* Columbus was of an engaging presence, tall, well formed and mus¬ 
cular, and of an elevated and dignified demeanor. His visage was long, 
bis nose aquiline, bis eyes light-gray, and apt to enkindle. His whole 
countenance bad an air of authority. Care and trouble bad turned bis 
hair white at thirty years of age. He was moderate and simple in diet and 
apparel, eloquent in discours, engaging and affable with strangers, and of 
great amiableness and suavity in domestic life. His temper was naturally 
irritable, but be subdued it by the benevolence and generosity of bis 
heart. Throughout bis life, he was noted for strict attention to the 
offices of religion; nor did bis piety consist in mere forms, but partook of 
that lofty and solemn enthusiasm, with which his whole character was 
strongly tinctured. Of a great and inventive genius, a lofty and noble 
ambition, his conduct was characterized by the grandeur of his views and 
the magnanimity of his spirit. The treatment which he finally exper¬ 
ienced from the Spanish court shows that ingratitude is not confined to 
Republics. 





172 


HAND-BOOK FOR OPTICIANS. 


had been passed in almost hopeless solicitation, amidst 
poverty, neglect and ridicule; the prime of his life had 
been wasted in the struggle, and, when his perseverance 
was finally crowned with success (Oct. 12th, 1492), he 
was about 56 years of age. 

Since the real shape and size of our earth is known, 
we are able to estimate the longest distance at which we 
can see an object, either with the naked eye or with the 
assistance of a spy-glass or telescope; because the range 
of vision is not dependent only on the acuteness of vision, 
or on the optical strength of an instrument, but is lim¬ 
ited also by the curvature of the surface of the earth. 
It is well known to all engineers that, on an even plane, 
only the head of a man is seen through a field- 
glass at the distance of three miles, and that, in order to 
see at a longer distance, either we have to take a higher 
stand, or the object must be raised to a greater height. 
Lighthouses are erected on this principle; the farther 
their light is to be seen, the higher they must be built, 
as ships have only a limited height. The following table 
shows both, the distance and the height at which a light 
can be seen. 

At 5 miles, the light must be 15 feet high. 


“ 10 

4 c 

4 4 

4 4 

60 

4 4 

“ 15 

44 

4 4 

4 4 

140 

4 4 

“ 20 

4 4 

4 4 

4 4 

250 

4 4 

“ 30 

4 4 

4 4 

4 4 

500 

4 4 

“ 42 

4 4 

4 4 

4 4 

1000 

4 4 


The rule for calculations of this kind is: “ The cur¬ 
vature of the earth is taken to be eight inches for the 
first mile, and increases according to the square of the 


bance.” 

For instance: 




2 miles, 

( 2 2 ), 

4X8 inches 

— 32" 

or 2§ feet. 

3 “ 

( 3 2 ), 

9X8 

4 4 

= 72" 

44 6 44 

4 “ 

( 4 2 ), 

16 X 8 

44 

=±= 

10 “ 

5 “ 

( 5 2 ), 

25 X 8 

4 4 

== 

16 “ 

10 “ 

(io 2 ), 

100 X 8 

44 

= 

66 “ 

15 “ 

(15 2 )- 

225 X 8 

4 4 

=== 

150 “ 

20 “ 

(20 2 ), 

400 X 8 

4 4 

= 

266 « 


The less height of lighthouses, as shown in the first 
table, is due to the elevated stand of the captain on 


RANGE OF VISION. 


173 


board the ship, which is supposed to be ten feet from 
the waterline, thus allowing the light to be somewhat 
lower.* 

Range of vision , practically, means “ the distance at 
which we are able to see.” On a plane surface, our 
vision is limited to three miles for all objects not higher 
than six feet; trees, towers and mountains are seen at a 
longer distance according to their height. But it is not 
only the height of an object that makes it visible, also 
its width must cover a certain space; besides, it makes 
a great difference if the atmosphere is clear or misty; 
if our eye is emmetropic or myopic; or if one object is 
more readily distinguished from its surroundings than 
another. The old rule that the width of an object must 
cover, at least, a visual angle of forty seconds, is super¬ 
seded by Snellen’s experiment with his test-types; and 
instead of seeing an object, which is not farther away 
than 5000 times its diameter, we have to shorten the 
range, according to his rule, to 3437 times. (See Chap. 
XII.) Thus we can find, approximately, the distance 
of any object, if we know its size; or its size, if we 
know its distance. The breadth of a man, on an aver¬ 
age, is eighteen inches (1J'); if we can barely see him, 
he is 3437 X 1£' away, or almost one mile. If he is 
dressed in white, and the surroundings are dark, the dis¬ 
tance may be set at 1| mile; if dressed in black, it may 
be only half a mile. 

If the back ground is dark, the impression of the 
different colors upon our eye range in the following order : 
White, yellow, orange, red, green, blue, violet and black, 
i. e., black disappears first, then violet, etc. White on 
black makes the strongest impression, and is seen the 
farthest. Upon a light-colored background the effect is 
the reverse, with the exception of violet, which disap¬ 
pears before red. 

* There is an analogy of the above rule in the calculation of the decreas¬ 
ing strength of light, when gradually removed from us; it also loses in 
power just in proportion to the square of the distance. The intensity, 
which a light has at one foot from us, diminishes in strength four times 
at two feet, and nine times at three feet. It requires, therefore, nine 
candles, three feet off, to produce the same amount of light as one candle 
produces at the distance of one foot from our eye. 



174 


HAND-BOOK FOR OPTICIANS. 


It is a well-known fact that all animals of prey bear 
the color of their hiding places.. This enables them to 
surprise their booty without being seen from any dis¬ 
tance. The striped tiger in the Indian swamps or jun¬ 
gles resembles the environs so perfectly that his victim 
is not aware of its presence till it is too late. The yel¬ 
low stems of the reeds, and the darker ground, produce 
a striking resemblance to the skin of this voracious beast. 
This curious play of nature is called “mimicry,” and 
benefits not only those beasts, but also many animals 
which are preyed upon. 

The hunter is thus sorely vexed, and often cannot 
make use of the above rule. But in military life there 
are many occasions where it is of an immense import¬ 
ance, by furnishing an estimate of the number of the 
advancing foe, and giving time to prepare for their 
reception. 

There arises another question analogous to the previ¬ 
ous one. I refer to the fact that it is not difficult to 
judge with any certainty the number of people congre¬ 
gated in large assemblages. The easiest way of this 
kind of calculation is to measure the ground in square 
feet, and divide the number by 4, as four square feet is 
ample room for a standing person. We can measure a 
space by walking over it and counting the steps. A full 
step (not a stride) measures on an average 2J'. Sup¬ 
pose, at a public meeting well attended, the bulk of the 
crowd extends in one direction 60 paces (150'); in an¬ 
other, 30 (75'); we have then 150 X 75 = 11,250 
square feet, divided by 4, gives 2812; and with the 
stray people counted in, we may estimate that about 
3000 people were attending the meeting. The next day 
we read in the different papers that the attendance was 
immense, and that there were at least 5000 persons 
present. Others may exaggerate the number even to 
10 , 000 . 


CHAPTER XXIV. 


Tears. 


The eyes of all vertebrates, with the exception of 
fishes and those amphibious animals that live in water, 
are provided with tear-glands, to moisten the surface of 
the eye and the inner side of the lids. If the tears 
were stopped, the outside of the eyeball would become 
dry and opaque, and sight be lost. As long as the tears 
flow they are drained through the tear-duct into the nose, 
and here mostly evaporate without any further annoy¬ 
ance. But, if in consequence of catarrh or any other 
cause, these tear-ducts are closed, the eyes fill with water 
which runs down the cheeks in the form of tears. This 
occurs in the eyes of animals as well as of men, but we 
cannot call it “ weeping;” it is only due to local causes. 

No animal weeps. Real weeping presupposes mental 
emotion , based on self-consciousness. Only human be¬ 
ings can reflect upon their own existence, and contem¬ 
plate themselves in an objective way. Without this 
great superiority over animals, we would be unable to 
touch that responsive chord of our spiritual existence 
which makes us weep for joy, grief or pain. 

Weeping* is synonymous to crying. Infants, when 
suffering even slight pain or discomfort, utter violent 
and prolonged screams; their eyes are firmly closed, so 
that the skin round them is wrinkled, and the forehead 
contracted into a frown. The mouth is widely opened 
with the lips retracted in a peculiar manner, which 
causes it to assume a squarish form. The firm closing 
of the eyelids and consequent compression of the eyeball, 
serves to protect the eyes from becoming too much 

* The verb “ to weep ” comes from Anglo-Saxon wop , the primary mean¬ 
ing of which is simply “outcry.”— “ Expression of Emotions in Man 
and Animals, by Charles Darwin, 1873.” 





176 


HAND-BOOK FOR OPTICIANS. 


gorged with blood. The contraction of the muscles 
(corrugator supercilii) surrounding the eye produces the 
transverse wrinkles across the forehead, whilst the con¬ 
traction of the pyramidal muscle ( pyramidalis' nasi ) 
causes the eyebrows to be drawn downward and inward, 
producing a frown.* The muscles surrounding the eyes 
are somewhat connected with those of the upper lip; if, 
therefore, the former are strongly contracted, those of 
the upper lip likewise contract and raise the lip. Even 
in grown persons, it is observed, that when tears are re¬ 
strained with difficulty, as in reading a pathetic story, 
it is almost impossible to prevent the various muscles, 
which with young children are brought into strong action 
during their screaming-fits, from slightly twitching or 
trembling. 

Infants, while screaming, do not shed tears or weep 
until they have attained the age of three or four months. 
This fact is most remarkable, as, later in life, no expres¬ 
sion is more general or more strongly marked than weep¬ 
ing. When the habit has once been acquired by an in¬ 
fant, it expresses in the clearest manner suffering of all 
kinds, both bodily pain and mental distress, even though 
accompanied by other emotions, such as fear or rage. 
With adults, especially of the male sex, weeping soon 
ceases to be caused by, or to express, bodily pain. This 
may be accounted for by its being thought weak and un¬ 
manly by men. The insane notoriously give way to all 
their emotions with little or no restraint; and it is ob¬ 
served that nothing is more characteristic of simple 
melancholia, than a tendency to weep on the slightest 
occasions. 

Weeping seems to be the primary and natural expres¬ 
sion of suffering of any kind. But common experience 
shows that a frequently repeated effort to restrain weep¬ 
ing does much in checking the habit. On the other hand it 
appears that the power of weeping can be increased through 
habit. A single effort of repressing tears is mostly inef¬ 
fective; indeed, it seems often to lead to an opposite re- 

* The pyramidal muscle is the fleshy part at the root of the nose, just in 
the straight line between the eyes, and is the chief support for the nose 
piece of Fox’s eyeglasses. 



TEARS. 


177 


suit. An old physician once remarked that the only 
means to check the occasional bitter weeping of ladies 
who consulted him, and who themselves wished to desist, 
was earnestly to beg them not to try, and to assure them 
that nothing would relieve them so much as prolonged 
and copious crying. 

The principal function of the secretion of tears is to 
lubricate the surface of the eye; also to keep the nostril 
damp, so that the inhaled air may be moist, and likewise 
to favor the power of smelling. But another important 
function of tears is to wash out particles of dust or other 
minute objects which may get into the eye. That this is 
of great importance is clear from the cases in which the 
cornea has been rendered opaque through inflammation, 
caused by particles of dust not being removed, in conse¬ 
quence of the eye and eyelid becoming immovable. The 
secretion of tears from the irritation of any foreign body 
in the eye is a reflex actiop; that is, the body irritates a 
peripheral nerve which sends the impression to the 
lachrymal glands. These glands cause the relaxation of 
the muscular coats of the smaller arteries,, which allows 
more blood to permeate the grandular tissue, thus induc¬ 
ing a free secretion of tears. When the small arteries of 
the face, including those of the retina, are relaxed under 
very different circumstances, for instance, during an in¬ 
tense blush, the lachrymal glands are sometimes affected 
in a like manner, for the eyes become suffused with 
tears. Cold wind, smoke, or a blow on the eye, always 
causes a copious secretion of tears. The glands are also 
excited into action through the irritation of adjoining 
parts; thus when the nostrils are irritated by pungent 
vapors, though the eyelids may be kept firmly closed, 
tears are secreted. Strong light has a tendency to cause 
lachrymation, especially when the eyes are diseased; 
the retina and cornea become excessively sensitive to 
light, and exposure even to common daylight causes 
forcible and sustained closure of the lids and a profuse 
flow of tears. When persons who ought to begin the 
use of convex glasses habitually strain the waning power 
of accommodation, an undue secretion of tears often 
follows. Children and silly persons very often cry 


178 


HAND-BOOK FOR OPTICIANS. 


because they set great value on trifling objects, whose 
refusal makes them extremely unhappy. 

Weeping is not always a sign of weakness, or an act 
to be ridiculed. The greatest men on earth had moments 
of mental agitation which made them weep; and while 
listening with awe to the story of their affliction, we 
unconsciously reach for our handkerchiefs to dry our 
eyes. We are overcome by a certain feeling, which is 
another prerogative of the human race—sympathy. The 
power of weeping is frequently a great blessing; it calms 
and cools our over-heated brain, and may prevent even 
serious incidents. As the opening of a valve saves the 
boiler from explosion, so tears gradually melt away that 
rock which rests upon our breast, and threatens to 
smother us by its insupportable weight. 


CHAPTEK XXV. 


Facial Expressions. 


The eye is the mirror of the soul, the reflector of 
mental emotions. Words can be misconstrued, but not 
the language of the eye. The discourse of an orator has 
a powerful ally in the eloquent expression of his eyes; 
under their influence the listeners are spell-bound, their 
heart echoes every sentiment which flows from his lips: 
he is the charmer, his auditory the enchanted prey. The 
eye is also the gate through which we can fathom the 
bottom of the mind, it reveals its secrets better than a 
lengthy discourse will do. — But, how is it possible that 
this simple organ has such marvelous qualities, and by 
what means are they effected ? Of course, the eyeball 
by itself cannot produce these wonders, but in connection 
with its surroundings, the eyelids and the eyebrows, it 
is the magic wand which, as the story goes, enervates 
the approaching lion, and compels him to make a cow¬ 
ardly flight before the majesty of the unflinching human 
eye.* The answer to this question is, that the expression 
of the eye is due, 

* The human eye has two distinctive peculiarities -which we do not find 
in animals: the eyebrows and the prominently visible portion of the white 
sclerotic coat. This last characteristic feature of the human eye is perhaps 
the principal cause that every animal turns its eyes down or sidewise, as 
we can readily observe when we take hold of the head of our dog, and look 
at his eyes. Although he may be very fond of us, he neyertheless tries to 
avoid the fixation of our look; and after freeing himself, he will jump 
about for joy, lick our hands, crouch down and is highly delighted with 
his release from that unbearable enchantment of our stare. This accounts 
for the above story, because, if anybody is accidentally confronted by a 
lion, it is natural that he is stunned, motionless, or paralyzed, with eyes 
wide open, thus involuntarily showing that “ominous white” to greatest 
advantage, which no animal can face without shrinking from its magic 
spell. Lion-tamers make use of this weakness in all animals; they keep 
those beast under the stern influence of their eyes, and awe them into 
submission, in spite of their superior brutal strength. 





180 


HAND-BOOK FOR OPTICIANS. 


1. To the changes of its surroundings, and 

2. to the position of its axis of vision. 

Of the part performed by the eyelids , that of the upper 
lid is the most important, because it is larger and more 
movable than the lower one. The raised, elevated lid 
admits free entrance of the full light, while the drooping 
lid shadows and darkens it. 

The eyes are wide open when we listen to something 
of great interest, which causes either surprise or alarm. 
Half closed eyes indicate indifference and indolence, and 
produce a dull and drowsy look. 

“Were his eyes open? Yes, and his month too. — 
Surprise has this effect, to make one dumb ; 

Yet leave the gate, which eloquence slips through, 

As wide as if a long speech were to come.” 

The eyelashes play also a great part in this respect. 
When they are long and fine, they impart to the eyeball 
a gentle and affectionate appearance, which the poets 
call “ the sweet pensive shadow.” 

“And eyes disclos’d what eyes alone can tell.” 

But when they are short and sparing, the look loses 
that mellowy appearance is rather unsympathetic, and 
gives the eye generally a cunning and sly air. 

The eyebrows are powerful organs of expression; we 
can produce a frown by Wrinkling and depressing the 
brows, while by elevating them we express incredulity, 
surprise, or contempt almost as plainly as by words. 
“Disdain and scorn ride sparkling in her eyes.” 

The position of the eye has another great effect; the 
so-called “ deep eye,” which is constantly shaded by the 
prominent forehead, makes a different impression from 
the “ shallow ” one. The deep eye is somewhat lacking 
the free motion of the upper lid, and as it is generally of 
a darker tint, besides being shaded by the projected 
forehead and eyebrow, it produces a determined, grave, 
often morose expression. 

“ Yet well that eye could fiash resentment’s rays, 

Or, proudly scornful, check the boldest gaze : 

Chill burning passion with a calm disdain, 

And with one glance rekindle it again.” 


FACIAL EXPRESSIONS. 


181 


The shallow eye, mostly light colored, shows to per¬ 
fection the innumerable variations of the human look. 
There is first the staring look; the eye is not fixed upon 
any distinct object, it is immovable and indicates hope¬ 
lessness, pain, fright, terror. The “ hopeless stare,” 
after the loss of all energy, is characterized by an indo¬ 
lent silence; the drooping of the upper lid produces 
the appearance of a weary despondency. 

“ In those sad eyes the grief of years I trace, 

And sorrow seems acquainted with that face .’ 5 

In cases of violent excitement, the staring look is the 
result of the convulsive exertions of the outside muscles 
of the eyeball, which, by acting all at once, push the 
eye forward. 

The look of the over-joyful is just the opposite; his 
eye is not staring in one direction, but is restlessly 
wandering from one object to another, because none is 
able to attract his attention long enough to counter¬ 
balance the inner excitement. By the quick changing of 
the position of his eyes, the observer receives constantly 
another reflection of them; such eyes sparkle with joy. 

“The joy of youth and health her eyes displayed, 

And ease of heart her every look conveyed” 

“ “While pleasure lights the joyful laughing eyes.” ” 

The color of the eye is of little importance in the dif¬ 
ferent expressions, but plays a prominent part at the 
time of courtship, when a lover goes into ecstasy over 
the color of the eyes of his beloved. 

“ Let other men bow, and utter the vow 
Of devotion and love without end, 

As the sparkling black eye in triumph draws nigh, 

Its glances upon them to bend. 

But give me the eye, thro’ which I can spy 
To the depth of a heart warm and true; 

Whose color may vie with the hue of the sky; — 

The soft, the sweet, love-beaming blue! ” 

All these changes of the eye are produced by the action 
of separate nerves. Some motions are controlled by our 
will, others are acted upon by the so-called sympathetic 
nerves which, for instance, regulate the dilatation and con- 


182 


HAND-BOOK FOR OPTICIANS. 


traction of the pupil, and produce other phenomena 
beyond our control. But by means of an “iron will”, 
or by long mental training, many expressions of the eye 
can be concealed from the observation of others. Skilled 
diplomatists, shrewd lawyers, professional swindlers, 
hypocrites and many more, often deceive others with 
sleek words and a trusting look; but the intelligent 
observer, instinctively, shrinks from their enticements, 
and is not easily caught by their deceitful schemes. The 
warning impression we receive as to the questionable 
truthfulness of their words is due to certain motions and 
positions of the eye, not controllable by their will. 
Their words do not touch the corresponding chord of 
our soul, they only cause dissonance and aversion of 
which we cannot give a clear account, but feel indi- 
stinctively. 

“ O what a tangled web we weave, 

When first we practice to deceive.” 

There are persons who win our affection without any 
effort, although their exteriors are plain, their features 
irregular, their discourse lacking eloquence and depth of 
knowledge, and yet we are fascinated by them. This is 
the witchery of an expressive eye, the reflex of an honest, 
sincere mind. 

“ In one soft look what language lies! ” 

A close observer of the facial expressions of different 
individuals will find a great variety in their delineation, 
based principally upon the direction of the axis of 
vision. In children this axis is almost constantly par¬ 
allel, producing the impression of thoughtlessness, or 
the childish innocent look. With increasing intelligence 
the eyes lose the parallelism by being fixed upon objects 
of investigation. All affections of the mind are now 
manifested by Certain motions and positions of the 
eyes, which become more and more convergent. The 
lurking look of the convict on trial, the watchful scrut¬ 
iny of the over-suspicious, the lustful look of the liber¬ 
tine, the piercing glance of anger, the rude gaze of the 
ruffian, and the fearful glare of the maniac; — all are 
modifications of the same act, produced by an increased 
convergency of the axis of the eyes. 


FACIAL EXPRESSIONS. 


183 


The gentle and refined affections of the mind restore 
to a certain degree the parallelism of the axis. It is this 
which appeals in the eye of the trusting; sparkles in the 
eye of the happy and the gay; subdues in the eyes of 
the affectionate and the loving; awes and elevates in 
upward gaze of piety and religion; or composes in the 
gentle regard of the devout and resigned. 

The eyes of a frightened person diverge; the wish 
to be far away from the place of danger causes the 
dilating of the pupils and the opening of the eyelids. 

In old. age the axis of vision again becomes parallel. 
The passions of former years are calmed, and the mind, 
in a contemplative mood, is now diverted upon its future 
distant home. At last the eye dies in the absolute par¬ 
allelism of the axis of vision. 


CHAPTER XXVI. 


History of the Invention of Spectacles, and the 
Gradual Development of the Optical Trade. 


Old tradition credits Phoenician merchants with the 
invention of glass. This nation occupied a part of the 
coast of Syria, between the Lebanon and the Mediter¬ 
ranean Sea, northw T est of Palestine, and was already 
widely known at the time of Jacob, the patriarch, about 
1750 years before Christ. But it seems glass was known 
before that time, as there has been lately found below 
the ruins of old Nineveh a lens evidently used for optical 
purposes. A knowledge of the manufacture of glass was 
early acquired by the Egyptians, who improved on it, 
and made even colored specimens. After the Romans 
conquered Egypt, this art was introduced into Italy, 
where they soon learned to make plate-glasS, and also 
produced a kind of glass which conid stand without in¬ 
jury the effect of hot fluids. They also claimed to have 
known a glass which was malleable, and to a certain degree 
unbreakable. A good story in relation to this states that 
a man once demanded to be brought before the Emperor, 
to whom he presented a goblet of glass. The Emperor 
was highly pleased with the splendid workmanship of it, 
but when it passed from hand to hand among the court¬ 
iers present, it accidentally fell to the floor, or, as it is 
also related, the artist himself threw it wilfully down. ( 
It did not break, but was badly dented. The man re¬ 
paired it immediately with a small hammer he had brought 
along with him. It is a pity that this important inven¬ 
tion is entirely lost. One Roman historian reports that 
Nero could not see very well, and that he made use of a 
large jewel in the shape of a lens, to enjoy a better sight 
of the fights of his gladiators. But this was not imitated 
by others, and is narrated by the historian only as one 




HISTORY OF THE INVENTION OF SPECTACLES. 185 

of the many strange extravagancies of this most remark¬ 
able man of the Roman empire.* 

The history of the invention of spectacles is closely 
connected with the general advance of science, especially 
as regards light . Light was a familiar phenomenon to 
the ancients, and from the earliest times we find man’s 
mind busy with tjie attempt to render some account of it. 
But without experiment, which belongs to a later stage 
of scientific development, little progress could be made 
in this direction. They satisfied themselves that light 
moved in straight lines; they knew also that these lines, 
or rays of light, were reflected from polished surfaces, 
and that the angle of incidence was equal to the angle of 
reflection. The first one who measured the refraction 
of glass and water at various angles, was Ptolemy, an 
Egyptian, about the year 150 P. C.; he states that the an¬ 
gle of refraction is always less than the angle of inci¬ 
dence. Nine hundred years later, the Arabian mathema¬ 
tician, Alhazen, wrote a valuable book on the reflection and 
refraction of light, containing also a description of the 
eye, and a philosophy of vision. Although he gives di¬ 
rections for making experimental measures of refraction, 
he does not furnish any Table of the results of such ex¬ 
periments. Vitellio, a Pole, about 1250, wrote an exten¬ 
sive work on optics, including such Tables, and asserts 
them to be derived from his own observations, which is 
very doubtful. 

We see that in the long period of eleven hundred 

* Nero was, perhaps, hypermetropic, but not myopic, as it is often 
stated. Myopia was not known in olden times, because it did not exist. 
Travellers have never found among uncivilized nations a case of myopia! 
People who do not read, or do not use their eyes for seeing small objects, 
are not near-sighted. In America, which is mostly an agricultural country! 
there are on an average twenty-five hypermetropics to"one myopic (cities 
excepted), while in Germany, where printing was invented, there are 
twenty-five myopics to one hypermetropic person. Myopia is often her¬ 
editary, but decreases in a few generations when the cause for it is 
removed. It is simply a temporary abnormality, and is usually acquired 
as the result of certain habits; it is without doubt of modern date. 

The causes of hypermetropia are not at all dependent upon the abuses 
of the eyes by reading or doing fine work. Many natural causes produce 
this abnormal condition, which is certainly as old as the human race, 
although it has been really understood and explained only in the present 
century. The close resemblance between myopic and hypermetropic eyes 
compelling both of them to use spectacles for near and far, has wrongly- 
caused many writers to make Nero near-sighted. 



186 


HAND-BOOK FOR OPTICIANS. 


years little progress was made in science, because the hu¬ 
man mind at that time took the opposite course of men¬ 
tal training. Instead of studying the forces of nature, 
and enjoying the bountiful gifts which an exceedingly 
friendly Providence had put within easy reach, people 
turned their eyes to the clouds till they lost sight of 
their beautiful surroundings. “ The men of the Middle 
Ages were so occupied with the concerns of a future 
world that they looked with lofty scorn on all things per¬ 
taining to this one. Notwithstanding its demonstrated 
failure during so many years of trial, there are still men 
among us who think the riddle of the Universe is to be 
solved by their appeal to consciousness. And, like most 
people who support a delusion, they maintain theirs 
warmly, and show scant respect for those who dissent from 
their views.’’ This is the reason why a man like Roger 
Bacon was hated and hounded to death, as happened to 
Galilei and other men of genius. — Bacon was a pro¬ 
fessor at Oxford, England; he made many wonderful 
discoveries in Optics as well as in Chemistry and Phys¬ 
ics, which were regarded by his ignorant contemporaries, 
especially by the jealous members of his own religious 
order, as the work of the devil, and caused his imprison¬ 
ment, at different times, for almost twenty years. He 
was the first to produce, or rather describe, a convex 
lens ,* but we do not find in his works the least hint that 
he combined these lenses into spectacles.! 

* A spectacle lens was discovered at Pompeii in 1854. This city was 
buried by an eruption of Vesuvius, in the year 79 P. 0., and was 
again discovered in 1748. 

-f- In his principal work, Opus Majus, he urges the necessity of a reform 
in the mode of philosophizing, and shows why knowledge had not made 
greater progress. It contains six parts: 

I. On the four causes of human ignorance. 

1. Authority (the force of unworthy authorities). 

2. Custom (the traditionary habit). 

3. Popular opinion (the imperfection of the undisciplined senses). 

4. Pride of supposed knowledge (the disposition to conceal our 
ignorance, and to make ostentatious show of our knowledge. 

II. On the source of perfect wisdom in the sacred scripture. 

III. On the usefulness of grammar (regarding correct translations). 

VI. On the usefulness of mathematics. 

V. On perspective. 

1. Organs of vision. 

2. Vision in straight lines. 

3. Vision reflected and refracted. 

4. Propagation of the impressions of light, heat, etc. 



HISTORY OF THE INVENTION OF SPECTACLES. 187 

He died in 1294, and only a few years afterwards this 
was accomplished in Italy. We may point to the year 
thirteen hundred as the one in which spectacles were in¬ 
vented, if we ignore the pretensions of the Chinese, who 
claim to have known them long before that time. How¬ 
ever this may be, all inventions made by them were bar¬ 
ren to the rest of mankind in consequence of their exclu¬ 
siveness. 

An old Latin document of the year 1303, found at the 
Convent of St. Catherine of Pisa, tells us that a monk, 
Alexander of Spina, who died in 1313, was so skillful a 
mechanic that he could reproduce any kind of work he 
had seen, or which had been described to him, and that 
he made spectacles after having seen them, and the in¬ 
ventor had refused to communicate the true process of 
their manufacture.* * This selfish inventor was probably 
Salvino Armato, on whose tombstone was the inscrip¬ 
tion : 

Qui Giace 

Salvino D’Armati Degli Armato 
Di Firenze, 

Inventore Degli Occhiali, MCCCXVII. 

Here Rests 

< Salvino, etc., Armato 
of Florence, 

Inventor of Spectacles, 1317. 


The use of spectacles spread very slowly, because peo¬ 
ple had little need of them. Only a limited number of 
men could read, books were very scarce and very dear. 
Printing was not yet invented, all books were written by 
hand, and it was only afterwards, when their circulation 
increased, that spectacles came into demand. An old 

VI. On experimental science. — 

This sixth part is undoubtedly the most remarkable portion of his work. 
It is indeed an extraordinary circumstance to find a writer of the thirteenth 
century, not only recognizing experiment as one source of knowledge, 
but urging its claim as something far more important than men had yet 
been aware of. 

* Ocularia ab aliquo primo facta, et communicare nolente, ipse fecit et 
communicavit. 


13 




188 


HAND-BOOK FOR OPTICIANS. 


chronicle of Nuremberg, in Germany, of the year 1482, 
mentions that there were several manufacturers of specta¬ 
cles in that city. 

Spectacles were for a long while merely objects of cu¬ 
riosity, and were made use of as a conspicuous novelty, 
as some years ago every “ dude,” male or female, had 
to wear blue glasses for fashion’s sake. In Spain they 
formed a part of the costume of every well-bred person. 
This absurd use of glasses was meant to increase the 
gravity of the appearance, and consequently the venera¬ 
tion with which the wearer of them was regarded. The 
glasses were proportional in size to the rank of the 
wearer. Those w r orn by the Spanish nobles were some¬ 
times three inches in diameter. The Marquis of Astorga, 
when having his bust sculptured in marble, particularly 
enjoined upon the artist not to forget his beautiful spec¬ 
tacles. 

After this first silly introduction of spectacles, they 
again fell into disuse for nearly three hundred years, dur¬ 
ing which no improvements deserving notice were made. 
How different were the people of that time from the pres¬ 
ent generation ! In less than no time we would have 
produced a concave lens also, and have presented the 
world with ft spy-glass. Think how quickly some years 
ago the Telephone was followed by the Phonograph; the 
one transmitting speech, the other reproducing it. I 
have searched in vain to find the name of the inventor of 
the concave lens; it was not in use long before the in¬ 
vention of spy-glasses, I think.* 

The'credit for the invention of spy-glasses, or tele¬ 
scopes, has been claimed by the friends of three parties: 
John Lippershey, Zacharias Jansen, both of Holland, 
and Galilei, of Italy. There is no doubt that Galilei 
first applied the telescope for observing the stars; but he 

* Mr. Child, an Englishman, has discovered lately in the Observatory 
at Pekin, China, an old astronomical telescope which was made in 1279, 
under the reign of Kublai Khan. Its mounting is cast in bronze, and is 
still well preserved. It was for four hundred years used as an ornament 
upon the terrace in front of the imperial palace, but was removed in 1670 
to the Observatory by order of the emperor Khang. A photograph of 
this antique instrument has arrived a few years ago in London. 



HISTORY OF THE INVENTION OF SPECTACLES. 189 


constructed his instrument after he had learned that by a 
combination of convex and concave lenses distant objects 
would appear much nearer. The real inventor of the tele¬ 
scope was without doubt John Lippershey, a spectacle 
maker at Middleburg, Holland. But according to Des¬ 
cartes, the inventor was Adrian Metiu#, who wrote on the 
17th of October, 1608, to the government of Holland, 
stating that he, as well as the spectacle maker of Middle¬ 
burg, was manufacturing the instrument “that brings dis¬ 
tant objects near.” Another document of Oct. 2d, 1608, 
lately found in the government archives, is the petition 
from Lippershey, praying for a thirty years’ patent on 
his invention. This was refused him because the instru¬ 
ment could not be used with both eyes at once; and after 
he had made a double one, the patent was again refused, 
because telescopes were then being made everywhere. 
But as a partial compensation for his disappointment, he 
received an order to construct for the government two 
binocular instruments, the lenses of which should be of 
rock crystal, and for which he was to be paid 900 guild¬ 
ers, about $300 a piece. 

The necessarily correct finish of lenses for telescopes 
gave a new impulse to the manufacture of spectacles, al¬ 
though they were still made in limited quantity by soli¬ 
tary workmen, and by hand. It is related of Spinoza, 
who died 1677, and who had learned the art of glass¬ 
grinding to make a living while writing his philosophical 
works, that he made a pair of spectacles for the cele¬ 
brated German philosopher, Leibnitz, who had formed 
his acquaintance at The Hague, Holland. 

The historical events which favored the development 
of this “ New Era of Science” were: 

The invention of printing, 1440; 
the discovery of America, 1492; and 
the gradual emancipation of the human mind from 
metaphysical dreams, 1517. 

The astronomers took the lead in the march of pro¬ 
gress, and the hitherto humble guild, or corporation of 
glass-grinders and manufacturers of spectacles, had to 
extend their former limited sphere to that of adroit me¬ 
chanics. They not only built telescopes and other com- 


190 


HAND-BOOK FOR OPTICIANS. 


plicated instruments used for scientific purposes, but also 
took personally an active part in the promotion of science 
by independent investigations and inventions. 

The hero who made the first scientific application of 
the new discovery was Galilei, but his telescope was very 
nearly the same as the modern single opera glass, being 
composed of one bi-convex objective lens and one bi-con¬ 
cave ocular lens. The theoretical explanation of this tele¬ 
scope was given by Kepler, 1611, who also suggested 
the use of a convex ocular lens, which allows a larger 
field of vision, but shows objects inverted. An instru¬ 
ment of this order was constructed by the capuchin, 
Anton De Rheita, 1645, and is called the astronomical 
telescope ; he afterwards added to the single ocular lens 
four separated convex lenses, thereby restoring the up¬ 
right picture, and called it terrestrial telescope . This 
monk also constructed a binocular telescope which was 
regarded rather as a thing of curiosity than of practical 
utility, until in modern days his plan has been accepted 
in opera glasses, microscopes, etc. The great defect of 
these instruments was their chromatic aberration, and to 
overcome this, enormously long telescopes were made. 
Huyghens, for instance, used an instrument of his own 
make, with an object lens of 123 ft. in focal length; 
which is still in the library of the Royal Society^of Lon¬ 
don. It incited the ambition of others to construct even 
longer telescopes; as Livini at Rome, Campuni at Bo¬ 
logna, and Auzout at Paris. It is stated that the latter 
made telescopes of from 300 to 600 feet focus, but they 
never could be used in practical observations. In these 
very long telescopes no tube w T as employed, and they 
were consequently termed aerial telescopes. Huyghens 
finally constructed one of 210 ft. long, but such instru¬ 
ments were unmanageable and soon went out of use. 
Besides, the increase of the aperture of object glasses 
could not altogether remove the coloration of the ima^e 
produced. 

Newton, who discovered the principle of the chromatic 
defect in lenses, maintained that the evil w T as irremedi¬ 
able, and that any combination of lenses could no more 
refract without producing color, than a single lens; he, 


GRADUAL DEVELOPMENT OF THE OPTICAL TRADE. 191 

therefore, constructed, 1671, a reflecting telescope. He 
is not the inventor of the reflector, as James Gregory is 
credited with its invention, but on account of Newton’s 
effort in its favor, it rapidly came into general use in 
England, being called the “Newtonian reflector,” in op¬ 
position to the “Gregorian,” manufactured afterwards 
by James Short and others. In 1718, Hadley made a 
mirror, six inches in diameter, with a focal length of 62 
inches and a magnifying power of 230 diameters. In 
1789, the elder Herschel constructed a reflector of forty- 
five feet length, with a speculum of four feet in diameter, 
with which he made wonderful discoveries. 

About the year 1747, Euler doubted the exactness of 
Newton’s proposition, and he declared that a combina¬ 
tion of lenses of different media would give a colorless 
image. The Swedish mathematician, Klingenstierna, 
contirmed the correctness of Euler’s suggestion by calcu¬ 
lation, and in 1757, John Dollond demonstrated it by in¬ 
venting the achromatic lens. It is said that Chester 
More Hall, was led by the study of the human eye, 
which he conceived to be achromatic, to construct an 
achromatic telescope as early as 1729, but kept his in¬ 
vention a secret. John Dollond and his son, Peter, con¬ 
structed achromatic telescopes of three feet, which pro¬ 
duced an effect as great as those on the reflecting prin¬ 
ciple forty-five feet long. Ramsden introduced, 1783, 
an eyepiece of two plano-convex lenses of equal focus, 
with their convex surfaces towards each other, and sepa¬ 
rated by a distance of two-thirds of their common focal 
length. By this arrangement, a flat field is gained, and 
the chromatic and spherical aberrations are so much re¬ 
duced as to be practicably imperceptible. 

The hope, now to construct instruments of unlimited 
size, was frustrated by the impossibility of obtaining 
large pieces of flint glass, and there was no material 
improvement in this direction for several years, till 
Fraunhofer, With the assistance of Francois Guinand, 
gave a new impulse to this branch of the optical business. 
Joseph Fraunhofer was born 1787, at Straubing, Bavaria; 
he was the son of a poor glazier, and was in his earlier 
years employed at the same trade. After his father’s 


192 


HAND-BOOK FOR OPTICIANS. 


death, 1799, he entered as an apprentice the establish¬ 
ment of a mirror-maker at Munich, where he had, 1801, 
the singular misfortune of being buried alive by the col¬ 
lapse of his boarding house. His miraculous rescue at¬ 
tracted the attention of the king, who made him a pres¬ 
ent of 18 ducats (about $40.00), for which he bought a 
machine to grind spectacle lenses. In 1806, he accepted 
the position as foreman in Utzschneider’s optical estab¬ 
lishment, where he soon became the greatest optician in 
Germany. His excellent telescopes and microscopes are 
known throughout Europe. Fraunhofer will always take 
a prominent place in the history of the optical trade; he 
was not only a practical and most skillful workman, but 
also a scientist of great renown. Still, a shadow darkens 
his fame, his selfish exclusiveness , which restrained him 
from making known, for the benefit of science, the true 
process of the manufacture of his perfected flint glass in 
large pieces. It was hoped that after his death some 
clue would be found among his writings; but, strange to 
say, his secret went with him into the grave.—His time 
can be considered as the beginning of the latest era in the 
development of the optical trade. The instruments for 
astronomical observation became an object of serious 
care. Extensive knowledge, intense thought, and great 
ingenuity were requisite in the astronomical instrument- 
maker. Instead of ranking with artisans, he became a 
man of science, sharing the honor and dignity of the as¬ 
tronomer himself. 

We now turn our attention from the telescope to its 
powerful rival, the microscope. The telescope had 
rudely dispelled Our self-conceited error, that the earth 
is the pivot on which the whole Universe revolves, by 
revealing myriads of new worlds, thus forcibly teaching 
the mortifying lesson of our own insignificance. The 
microscope, acting as an antidote to the former, again 
restored our smallness to a state of gigantic greatness; it 
revealed a world of hitherto invisible wonders of nature, 
and clearly manifested that everything, great or small, is 
equally marvelous. These two instruments have done 
more for the enlightenment of men than any invention 


GRADUAL DEVELOPMENT OF THE OPTICAL TRADE. 193 

before or since. The invention of the simple microscope 
is not claimed by any one; we do not know the inventor. 
The earliest magnifying lens known, if indeed it was 
used for this purpose, is the rude one found by the Eng¬ 
lishman, Layard, in the palace of Nimrud (at Nineveh); 
it is made of rock crystal, and is far from being perfect. 
Aristophanes tells us that burning spheres were sold in the 
shops at Athens, about 400 years B. C. There is no evi¬ 
dence that lenses were used at this early date for magnify¬ 
ing purposes, but, instead of them, glass globes filled 
with water, which Seneca alludes to, were employed. 

It is not until the seventeenth century that we find 
powerful magnifiers of glass, actually employed for sci¬ 
entific investigation. Most of the magnifiers used by the 
early observers were minute single lenses of glass, often 
small spheres formed by melting threads of glass. The 
small single lenses of high power are usually plano-con¬ 
vex, the plane side toward the object. Upon David 
Brewster’s suggestion, lenses were ground by Peter Hill, 
a skillful optician of Edinburgh, and by Pritchard, of 
London, of garnet, sapphire and diamond.* 

The garnet lens was found superior to all others, being 
free from double refraction, and even superior to glass. 
Brewster also invented a very powerful single microscope, 
known as the Coddington Lens , which consists of a 
sphere with a deep concave groove cut around it, and 

* Some years ago two opticians of Paris, Treconrt and Oberhauser, laid 
before the Parisian Academy lenses of the diamond, sapphire and ruby, 
which were used in connection with glass lenses in microscopes, but they 
had no advantage over glass. A letter from David Brewster, lately pub¬ 
lished, explains the cause of the failure. He says of his own experiment, 
above mentioned: “ The diamond, before it was worked, had all the ap¬ 
pearance of internal brilliancy; but, after being polished, it presented a series 
of stratified shades, which rendered it useless for the required purpose. I 
afterwards learned that lapidaries were acquainted with this appearance, 
and were unwilling to take the risk on themselves of cutting up diamonds 
- for optical purposes. On a minute examination of this phenomenon, it 
appeared that these different shades occurred in regular strata, each section 
being about the one-hundredth part of an inch, and each stratum having 
a different focus, and being of a different degree of hardness and specific 
gravity. The inferences drawn from the above facts were: that the dia¬ 
mond was a vegetable substance, and that its parts must have been held in 
solution and subjected to different degrees of pressure at the different 
stages of existence. If, on the contrary, it was of mineral origin, as is 
generally believed, it would be subject to the laws of crystallization, and 
its crystals would necessarily be homogeneous and not stratified. 



194 


HAND-BOOK FOR OPTICIANS. 


blackened so as to shut oft the marginal pencils of light, 
thus giving a wider field and a more perfect image of the 
object. In the Stanhope Lens , the curvatures are un¬ 
equal, but its magnifying power is so strong that a drop 
of water may be examined by applying it to the less 
convex, or plane surface. 

In the construction and use of lenses two great diffi¬ 
culties present themselves. It is practically almost im¬ 
possible to make small lenses with any other than spherical 
curves, and unfortunately simple spherical lenses do not 
bring the rays to a perfect and exact focus. If it were 
possible to construct lenses with elliptical or hyperbolic 
curves, the spherical aberration would be avoided; but 
even then, since the different rays of the spectrum are re¬ 
fracted differently, the focal length for red light would 
be greater than for blue, and it would be impossible to 
obtain a sharp image free from chromatic aberration. 
In order to overcome these difficulties, doublet and triplet 
lenses were invented and introduced, which led gradually 
to still greater combinations, till the simple microscope 
was transformed into a compound one, now the only in¬ 
strument used for minute researches. The theoretical and 
practical difficulties that had to be overcome in develop¬ 
ing the best modern compound microscope from its 
embryonic condition were so great that, until within the 
last seventy-five years, the very possibility of success 
was doubted by the highest authorities in optical science. 

The manufacture of microscopes was much favored in 
England. Since the time of Ramsden, there has been an 
industrious contest among the English opticians in per¬ 
fecting that instrument more and more, and they were 
greatly encouraged by the liberal support of the English 
people. There is hardly a college or school without it, 
many ships carry a good instrument, even private studios 
and parlors are supplied with the luxury of an improved 
microscope. Among the many skillful opticians I may 
mention: Wenham, Swift, Parkes & Son, Stephenson, 
Smith & Beck Bros., Powell & Lealand. — Other nations 
were not so liberal in their support; for instance, the 
French opticians were chiefly dependent on the export of 
their instruments, and although they did not keep step 


GRADUAL DEVELOPMENT OF THE OPTICAL TRADE. 195 

with the English manufacture, still some opticians made 
quite a reputation for themselves; as Chevalier, Nachet, 
Oberhauser, Hartnack, etc. Since the time of Fraunhofer, 
the German and Italian opticians also produced fine instru¬ 
ments which could be favorably compared with the best 
English microscopes. There was Amici at Modena, 
G. & S. Merz at Munich, S. Ploessl at Vienna, C. Zeiss 
at Jena, and others of great ability. — America was for 
many years a profitable market for European instruments, 
but since Chas. A Spencer, Robert B. Tolies and others, 
we can fully compete with the old world as regards 
telescopes and microscopes. Only in the manufacture 
of Opera Glasses we are still in our infancy, although 
the demand for them is such that they form an impor¬ 
tant article of manufacture, of which Paris is the great 
seat. So largely and cheaply are they produced in 
Paris, that it has nearly a monopoly of the trade. They 
can be bought from 75 cents up to $30.00 a piece. The 
cheapest opera glasses consist of single lenses; those of 
the better class have one compound achromatic lens. A 
very ordinary construction for a medium price is to have 
an achromatic object-lens, and a single eye-lens. In the 
finest class of opera glasses, both the eye-lenses and 
object-lenses are achromatic. Ploessl’s celebrated field- 
glasses (Feldstecher) have twelve lenses, each object- 
lens and eye-lens being composed of three separate lenses. 

Almost every inventor and scientific discoverer has laid 
claim on our dexterity to execute his idea ; Wollaston 
came with his Camera Lucida, Wheatstone with his 
Stereoscope, Daguerre with his Photographic Camera, 
Faraday with his Electric Machines, Morse with his 
Telegraphic appliances, Kirchhoff and Bunsen with their 
Spectroscope, the Sugar-Industry with its Polariseope, 
Helmholtz with his Ophthalmoscope and the Oculists with 
their Compound Lenses. Indeed, we have to make in¬ 
struments for Electricians, Surveyors, Navigators, 
Astronomers, Chemists, Physicists, Meteorologists, etc.; 
but it is only within the last century that our trade has 
risen to that great prominence it occupies to-day. We 
are now an indispensable factor in scientific pursuits, 
and furnish instruments, not only the most scientific, 


196 


HAND-BOOK FOR OPTICIANS. 


but also the most useful ever offered to benefit the world. 
We have reason to be proud of our achievement, but 
we must not forget that we were merely the tools, 
executing the order of scientists, who did the brain work 
for us and that we have not many opticians like Fraun¬ 
hofer and Chas. A. Spencer to boast of. 

The spectacle business advanced considerably after the 
oculists detected the asymmetrical refraction of the cor¬ 
nea, called Astigmatism. Thos. Young, of England, 
made the first studies in astigmatism in 1783, but it was 
little noticed by his contemporaries. It was only after 
Donders, Helmholtz, Graefe, Javal, Knapp, and others, 
more than fifty years afterwards investigated it, and 
explained the method of its correction by means of 
cylindrical lenses , that it was generally understood. The 
manufacture of such cyl. lenses with all their combina¬ 
tions, and especially their correct setting , was a new 
departure in our trade, and many opticians were con¬ 
siderably troubled before they fully mastered the diffi¬ 
culties in connection with this most delicate correcting 
medium in the shape of spectacles. A competent opti¬ 
cian of 1860, falling asleep like Rip Van Winkle , and 
awaking to-day, could not fill the simplest order of an 
oculist, but would have to learn his trade over again. 

As long as the selection of spectacles was left to the 
opticians, they contented themselves with the correction 
of a limited number of defects, and declared the remain¬ 
der incurable. They did not know the nature of irregu¬ 
larities, such as Hypermetropia or Astigmatism, and 
were, therefore, totally in the dark about their correc¬ 
tion. Oculists formerly considered it beneath their dig¬ 
nity to concern themselves with spectacles, and after 
they had restored the injured or suffering eye to a 
healthy state, they turned the patient over to an optician 
for the proper selection of glasses, unconcerned whether 
his selection was a good or bad one. It is only since 
prominent oculists investigated such “incurable” cases, 
that they can be thoroughly corrected by spectacles. 
Although they are manufactured by opticians, the credit 
of their beneficial action belongs to those eminent ex¬ 
plorers who gradually wrenched their selection from the 


GRADUAL DEVELOPMENT OF THE OPTICAL TRADE. 197 

hands of mostly indifferent mechanics, who, destitute of 
the necessary scientific education, have to content them¬ 
selves at present with a secondary position under the 
leadership of the oculists. There is no blame attached 
to our present position, as it is not at all a step backward. 
On the contrary, the standard of our trade has advanced 
considerably, but it has not kept step with the gigantic 
progress of Ophthalmology, which has no equal in medi¬ 
cal history. In the last thirty years Ophthalmology and 
general Surgery have become exact sciences, while the 
rest of medicine is yet for the most part empirical, as 
was the case with our mechanical and hap-hazard manner 
of selecting spectacles, when the patient was the princi¬ 
pal judge of their correctness. 

The selection of spectacles in complicated cases is now 
extensively practiced by oculists, who are, as physicians, 
qualified to prepare the eye for a thorough examination. 
Any optician, tampering with the eyes of an easily fright¬ 
ened customer, may cause himself great trouble if he 
cannot legally attach to his name an M. D. Only cases 
of simple presbyopia, manifest myopia, hypermetropia, 
and some cases of astigmatism, may be properly investi¬ 
gated by an optician, because the other and more compli¬ 
cated errors of refraction require that the ciliary muscle 
be temporarily paralyzed by a mydriatic, and that in this 
state of the eye accurate and repeated measurements be 
made with test-types and trial lenses. Signs in the win¬ 
dows of opticians which read: “ Examination of the eyes 
made free of charge,” smack of quackery, and should be 
removed. 

If I now allude, briefly, to the part America has taken 
in the general development of the optical trade, I have to 
draw the attention of the reader to the well known fact 
that we had during the colonial time no industry worth 
mentioning; we simply exchanged our natural and agri¬ 
cultural products for English manufactures. Optical 
goods were still imported from Europe long after the es¬ 
tablishment of our political independency from England. 
—The first one mentioned in this respect is Godfrey, 
who was, after all, no optician but a glazier; he invented 
the sextant. He was of Philadelphia, (for many years 


198 


HAND-BOOK FOR OPTICIANS. 


the headquarters of our slowly developing optical indus¬ 
try) ; so was Eittenhouse, McAllister, Queen, Saxton, 
Zentmayer, etc. Other states soon followed in the path 
Pennsylvania had so ably opened; especially New York, 
with a fair line of opticians and inventors, like Fitz, 
Wales, Grunow, Spencer, Draper, Prentice, Fassolt, etc. 
Massachusetts was conspicuously represented by Tolies 
and Alvan Clark; even a Southern state by Eiddell. At 
present nearly every Northern, Middle and Western 
state can boast of some competent opticians. Since the 
last fifteen years we manufacture all frames for specta¬ 
cles and eyeglasses, and also have commenced lately to 
grind our own lenses; I mention in this respect, Bausch 
& Lomb in. Koch ester, the American Optical Co. in 
Southbridge, and the Katonah Optical Co. 

In concluding this chapter, we must bear in mind that 
when we come to a great man who discovers or lays down 
new laws, there have always been a number of less known 
observers who have collected the facts from which he 
has formed his conclusions. Every country contributes its 
share to the development of science; we may, therefore, say 
that science is international. Its achievements are open to 
the world at large, and the readiness with which any 
nation accepts and introduces them shows its average 
intelligence. A superficial comparison of the most prom¬ 
inent discoveries contributes greatly to discriminate 
between the special characters of the different nations. 
France , for instance, excels in inventions for enjoying 
and beautifying life, thereby showing the happy dispo¬ 
sition to take life mostly from the rosy side; German 
inventions bear, a more scientific, yea serious aspect, 
indicating a rigorous submission to life’s sobriety ; 
England , the favorite foster-child of the world, prances 
proudly in the general race of progress, but with a sig¬ 
nificant wink to realistic ends; and—America follows her 
example. It individualizes each step of progress by the 
distinction of a patent, which is by no means an impedi¬ 
ment to progress, but, on the contrary, a fruitful cause 
of many important inventions. America is the foremost 
advocate of this doctrine, and is benefited by it to such 
an extent that at present our telescopes, microscopes, 


GRADUAL DEVELOPMENT OF THE OPTICAL TRADE. 199 


and above all, our spectacles can stand a fair comparison 
with the best European manufacture. Fifty years ago, we 
still imported all optical instruments and appliances 
from England, France or Germany: but of late we only 
import the optical glass , and do to a good extent the 
grinding of lenses here as well, or even better, than 
they formerly did in Europe. If our glass-industry had 
advanced in the same proportion as the other branches of 
the optical trade, we also could expect in the near future 
an emancipation from the further importation of that 
article. 


Different Names for Spectacles. 

The English word Spectacles is the plural form of 
spectacle, which is derived from the Latin noun Specta- 
culum , a sight, a show, and is formed from the verb 
spectare , to look at; to behold. 

The French word Lunettes is also the plural of lunette, 
which means a little moon, a “ moonlet, ” referring to 
the round shape of spectacle lenses. 

The German word Brille , like the Dutch Bril, and the 
Danish Briller , is derived from Beryl, a transparent 
green-bluish mineral," called by the jewelers Aqua Marine. 
In former years people in Germany called all colored 
glass Berylle, and as a great many spectacles, especially 
those worn for fashion’s sake, were set with plain colored 
glasses, this optical instrument received its name from 
that mineral. The Latin name for it is berillus , the 
fundamental idea of which denotes a shining or sparkling 
mineral substance, a crystal or crystal-like glass. The 
noun brilliant , now used only in reference to diamonds, 
is derived through the medium of the French word briller , 
to shine, to glitter, to sparkle (present participle, bril- 
lant ). 

Italians say Occhiali; occhio = eye. 

Spaniards say Ante ojos; ante = before, ojo = eye. 

Portuguese say Oculos , eyes. 

Modern Greeks say Dioptres. 

Poles say Ohulary. 



200 


HAND-BOOK FOR OPTICIANS. 


.Swedes say Glas-ogen , glass-eyes. 

Russians “ Ozku (atschkiii); Otsko = eye. 
Roumanians say Ochilary (ot-chee-la-re). 
Hungarians “ Pajpaszem. 

Turks say Guzlegun. 

Hindoos say Chasrna (tchasma), frame. 
Hebrews “ Sechuchis V Ay in. 

Chinese “ Nong-Kieng , eye-glass. 
Japanese “ Megsme , eye-mirror; and in 
Volapiik, we say Lxm. 


CHAPTER XXVII. 


Prominent Opticians, Scientists and Inventors. 


“It is the commendation of a good huntsman to find 
game in a wide wood, but it is no imputation 
if he has not caught all. ” Plato. 

Airy , Geo. B., born 1801, an English astronomer, 
first at Cambridge (1828), and since 1835, at the Green¬ 
wich Observatory. He has deservedly the reputation of 
being one of the most able and indefatigable of living 
scientists. His important contributions to astronomy, 
magnetism, meteorology, and other sciences are contained 
in leading cyclopaedias and in the annals of learned socie¬ 
ties . He introduced several new astronomical instruments, 
among them the water-telescope, the transit-circle, and 
the large equatorial erected from his plans in 1859. He 
published, 1851, 44 Six Lectures on Astronomy ” ; in 1866, 
44 The Undulatory Theory of Optics ; in 1869, 44 On 
Atmospheric Chromatic Dispersion, ” etc. He made 
many researches in physics and optics, and is the inven¬ 
tor of cylindrical lenses for the correction of astig¬ 
matism. 

Alhazen , Abu Ali (died 1038 at Cairo, Egypt), was a 
great mathematician, and the first notable discoverer in 
optics after the time of Ptolemy. To him is due the 
explanation of the apparent increase of heavenly bodies 
near the horizon; he also taught that vision does not 
result from the emission of rays from the eye, which 
was the favorite theory for many centuries before and 
after him. He wrote a book on the refraction of light, 
especially on atmospheric refraction, showing the cause 
of morning and evening twilight. Only two of his works 
have been printed, his 44 Treatise on Twilight,” and his 
44 Thesaurus Opticse,” or collection of optical facts. 




202 


HAND-BOOK FOR OPTICIANS. 


Amici , G. B. (1784-1863), a celebrated optician and 
astronomer at Modena, Italy; constructed the best reflec¬ 
tors and greatly improved achromatic microscopes. He 
invented and perfected also different kinds of camera- 
lucida for drawing purposes. 

Arago, D. F. (1786-1853), celebrated French physi¬ 
cist; discovered the colored rings of crystallized plates 
in polarized light. Upon this discovery is based the 
principle of the “ polarizer” for testing pebbles. 

Archimedes (287-212 B. C.), the most celebrated 
ancient mathematician; invented the hollow “Archim¬ 
edes’ Screw,” a machine for raising water. He discov¬ 
ered the problem that a solid body, immersed in water, 
loses so much of its weight as the water would weigh 
which is removed by the body (specific gravity). In 
defending his native city^, Syracuse (Sicily), against the 
Roman fleet under the command of Marcellus, he is said 
to have made use of powerful burning mirrors. 

Argand , A. (1750-1803), a Swiss chemist; invented, 
1782 a lamp called after himself. The wick has the 
form of a hollow cylinder, through which a current of air 
ascends, so that the supply of oxygen is increased. This 
contrivance prevented the waste of carbon, which in the 
old lamps escaped in the form of smoke, and it greatly 
increased the amount of light. He also added the glass- 
chimney, by which a draft is created and the flame ren¬ 
dered more steady. 

Bacon , Roger (1214-94), studied at Oxford and Paris, 
where he received the degree of Doctor of Theology. 
After his return to England, he accepted a professorship 
in the University of Oxford. Here he joined the broth¬ 
erhood of the Franciscans, and was termed by his 
brother monks “ Doctor Mirabilis.” His science and 
philosophy was almost universal, embracing Mathe¬ 
matics, Mechanics, Optics, Astronomy, etc. He made 
many discoveries, or had some knowledge of the most 
remarkable inventions which were made known soon 


OPTICIANS, SCIENTISTS AND INVENTORS. 


203 


afterwards. His principal work “Opus Majus,” was 
addressed to Pope Clement IV ( 1265-68), who was 
formerly Legate to England, and who admired the talents 
of the learned monk, and pitied him for the persecution 
to wdiich he was exposed.—The influence of Bacon upon 
his contemporaries was not great; he was suspected of 
magic and was placed several times in close confinement 
in consequence of this charge, once for ten consecutive, 
years (1268-78). ** 

Barlow , Edward (1639-1719), an English mechanician, 
invented, 1676, the repeating clock and watch. 

Baume , Antoine (1728-1804), a French chemist. His 
areometer, also called according to its applications hydro¬ 
meter, saccharometer, etc., made him widely known. It 
is still in use for measuring the specific gravity or density 
of different liquids heavier or lighter than water. 

Biot, J. B. (1774-1862), celebrated French mathe¬ 
matician and physicist; studied with success the dis¬ 
covery of Arago, and published,, some important re¬ 
searches about polarization and double refraction. He 
still defended Newton’s emission theory of light. 

Boulton, Matthew ( 1728-1809 ), a skillful English 
machinist; inherited from his father an extensive steel 
manufactory, which he changed into a manufactory of 
steam engines, after he had associated himself with the 
penniless optician, James Watt. The improvements of 
steam engines were the joint efforts of both, although 
they are now chiefly credited to the genius of the latter. 

Bradley , James ( 1692-1762 ), an eminent English 
astronomer, was 1721 appointed professor of astronomy 
at Oxford. In 1727, he announced the important dis¬ 
covery of the aberration of light, which serves to demon¬ 
strate the earth’s motion around the sun. In 1741, he 
became the successor of Halley at the Observatory of 
Greenwich. His greatest discovery was in 1747; he 
found that the relation of the earth’s axis to the ecliptic 
is not constant, a fact which explained the precession of 
the equinoxes and the nutation of the earth’s axis. This 
discovery forms an important epoch in astronomy. 


14 


204 


HAND-BOOK FOR OPTICIANS. 


Bramah , Jos. ( 1740-1814 ), an English mechanic ; 
invented the “ hydraulic press ” (1795). 

Brandt , Geo. ( 1694-1768 ), a Swedish chemist and 
mineralogist; discovered, 1733, the metal Cobalt, now so 
extensively used in the manufacture of blue lenses. 

Breguet , A. L. (1747-1825), celebrated French mech¬ 
anic, made many important inventions in watchmaking 
as well as in physics. He invented the metal thermo¬ 
meter which consists of a thin strip of metal, composed 
of three layers, of silver, gold and platinum. This 
strip is curled up into a helix, the silver being outermost. 
As the temperature rises the silver expands more than the 
gold and the gold more than the platinum, and the helix 
coils itself up; in lower temperature it acts the opposite. 
The end of the helix carries an index by which its rota¬ 
tion is made manifest. 

Breisig , professor at Danzig, Prussia, invented the 
panorama. The first public exhibition was made, 1787, 
in Edinburgh, by Robert Parker. 

Brewster , Sir David (1781-1868), celebrated English 
physicist; made great discoveries in the polarization of 
light and in double refraction of crystals ; invented 
the “ kaleidoscope,” and described the Coddington lens. 
In 1832, he published his “Treatise on Optics,” wrote 
many valuable articles for the “Encyclopaedia Britun- 
nica,” and was one of the last defenders of the “ emission 
theory.” He is called the “ Father of Modern Experi¬ 
mental Optics.” 

Bunsen , R. W., was born 1811, professor of chemistry 
in Germany; invented a burner which bears his name. 
In 1860 he invented the magnesium light which has 
proved so important in photography. The greatest dis¬ 
covery with which his name is associated, is that of the 
“spectrum analysis,” made in conjunction with his 
friend, Kirchoff, which has been the means of working 
so many wonders in chemistry, and revealing so much to 
astronomers. 


OPTICIANS, SCIENTISTS AND INVENTORS. 205 

Celsius , A. C., (1701-44), a noted Swedish astrono¬ 
mer; divided the scale of the thermometer into one hun¬ 
dred equal parts, from the freezing point of water to its 
boiling point, in opposition to Reaumur and Fahrenheit. 

Chevalier , Arthur, born 1830; inherited, 1859, the 
large optical establishment at Paris from his father, Charles 
Chevalier. He, as well as his father, has made many im¬ 
provements in the appliances for microscopes and other 
optical instruments. He published several instructive 
books ; “ The Art of an Optician,” “ The Student of the 
Microscope,” “ The Student of Photography,” “ Hand¬ 
book of the Oculist Student,” (Manuel de l’Etudiant 
Oculiste), etc. 

Clark , Alvan, ( 1804-87), of Cambridgeport, near 
Boston, is the most eminent manufacturer of telescopic 
lenses. He is a self-made optician, had never seen a 
lens ground; was formerly an engraver and portrait 
painter, but began, 1844, to study technical optics and 
astronomy in order to assist his oldest son, George B. 
Clark, a student at Andover, in his studies as engineer. 
Both, father and son, experimented in making a reflecting 
telescope, and succeeded so well that they continued, 
and gradually established a reputation here and in Eng¬ 
land. After his second son, Alvan G. Clark, a practical 
machinist, joined the establishment, they tried to con¬ 
struct “refractors,” and increased their lenses to sizes 
unknown before. In 1860, they constructed a telescope 
with a lens of eighteen inch diameter, and sold it to the 
Astronomical Society of Chicago. Up to that time, fif¬ 
teen inches had been the diameter of the largest lens in 
the world.—During the war they were kept busy making 
binocular field glasses for the army, but soon resumed 
the manufacture of telescopes. In 1871, they constructed 
a telescope for the Naval Observatory at Washington, 
with an objective lens of twenty-six inches in diameter; 
they also made a duplicate of it for the Lee University 
of Virginia. The next great telescope was made for the 
Russian Observatory at Pulkowa; it has a clear aperture 
of thirty inches, a focal distance of 45 feet, and a 


206 


HAND-BOOK FOR OPTICIANS. 


magnifying power of 2000 diameters. But the greatest 
triumph of their technical skill is the new telescope of 
thirty-six-inch diameter for the Lick Observatory of the 
University of California. — He made several discov¬ 
eries; he invented a double eyepiece, and devised a very 
accurate method of measuring small celestial arcs. 

Coddington , Henry (died 1845), an English mathema¬ 
tician; published, 1829, a valuable book in two parts 
“System of Optics.” In 1830, he published an essay 
“On the Improvements of Microscopes,” in which he 
strongly recommended the “ grooved sphere ” lens (first 
described by Brewster in 1820), which by his recommen¬ 
dation was brought into general use under the name of 
“Coddington Lens.” 

Cooke , Thomas (1807-68), of York, England; w r as 
originally a shoemaker in a small country village, but 
at the age of seventeen opened a school and in his lei¬ 
sure taught himself geometry and mathematics. His 
ambition was to construct a reflecting telescope, which 
led him to grind and polish lenses and specula, and with 
great perseverance and rare skill he accomplished his 
purpose. He then studied the optical laws of refraction 
in order to make an achromatic refractor; he constructed 
one of four inches, which had an admirable defining 
power. This telescope established his name as an opti¬ 
cian ; he gave up teaching and took to telescope-making. 
He opened, 1836, a shop in York, added to it the business 
of a general optician, his wife attending to the sale in 
store, while he was working in the back-room on tele¬ 
scopes. With the assistance of his brother as grinder, 
and his sons as mechanics, he erected in 1855 a com¬ 
plete factory. His work was always first-rate, and 
became known all over the world. In the same year, 
at the first Paris Exposition, his six-inch equatorial tele¬ 
scope was awarded the highest prize, a silver medal. _ 

He turned out many telescopes, but the largest had only 
an aperture of ten inches, while Merz & Mahler, of Mun¬ 
ich, made some of fifteen inches, and Alvan Clark, 1860, 
one of eighteen inches. Cooke was too ambitious not 


OPTICIANS, SCIENTISTS AND INVENTORS. 207 

to try to beat them; he, therefore, commenced to make 
one of twenty-five inches, but before it was mounted, 
his health broke down, he died from mental anxiety and 
over-work. Many call him the “English Fraunhofer.” 

Copernicus , Nic. (1473-1543), was the founder of 
modern astronomy; demonstrated that the sun was the 
center of our system. Up to his time it was taken for 
granted that the earth was the center of the Universe, 
and that the sun with the planets, and all the stars were 
moving around it. His theory was received with the 
same opposition as, one hundred years later, Huyghen’s 
undulatory theory of light. The strongest opponent was 
the astronomer Tycho Brahe, and the Church, which 
persecuted all prominent defenders of this theory. (See 
Galilei.) 

# Cronstedt , A. F. (1722-65), a Swedish mineralogist; 
discovered, 1751, the metal Nickel. 

Daguerre , L. J. M. (1789-1851), a French painter, 
known as the inventor of the present photography. His 
pictures were called “ Daguerreotypes,” after his name, 
and were first exhibited at the Paris Academy by Arago, 
1839. This invention compelled the optical trade to 
manufacture the Camera Obscura. 

Dalton , John (1766-1844), celebrated English physi¬ 
cist, and founder of the atomic theory of chemistry. 
He was the first who published facts about color-blind¬ 
ness, called foolishly after him “ Daltonism.” 

Descartes , Rene (1596-1650), known also by the Latin 
name “ Cartesius ” ; was the most remarkable philoso¬ 
pher and greatest mathematician of his age. His 
“ Dioptrique, ” published in 1639, is an everlasting 
monument to his talent and acuteness of mind. He 
demonstrated that the aberration of a spherical lens 
would be considerably diminished by increasing the con¬ 
vexity of its axis, viz. by changing the spherical curve 
into a parabola. He also proved (1637) that the image 
formed upon the retina is inverted. He is the father of 
modern philosophy, and the founder of analytic geometry. 


208 


HAND-BOOK FOR OPTICIANS. 


Dollond , John (1706-61), an English optician, well 
versed in mathematics; was a silk-weaver in his youth, 
and employed his leisure hours in the study of science. 
He invented the achromatic telescope, for which he re¬ 
ceived the Copley medal from the Royal Society of 
London (1758). 

Dollond , Peter (1731-1820), improved upon his fa¬ 
ther’s efforts, in conjunction with his brother-in-law, 
Ramsden. He published an “Account of the discovery 
of refracting telescopes ” (London, 1789). 

Bonders , F. C. (1818-89), a Dutch physician, studied 
at the University of Utrecht ; practiced first at The 
Hague, then established at Utrecht an institution for 
treating diseases of the eye. His principal works are: 
“ Study of the Movements of the Eye,” 1847; “ Astig¬ 
matism,” 1862; “Anomalies of Accommodation and 
Refraction of the Eye,” 1865. His researches regarding 
Hypermetropia and Astigmatism created a new epoch in 
Ophthalmology, and although he was ably assisted by 
independent discoveries, in this line, b} r different co-la¬ 
borers, his name will forever brightly shine in the annals 
of optical science as a benefactor to mankind, and as an 
original scientific investigator. 

O O 

Drummond , Thomas (1797-1840), a Scottish engineer; 
invented, 1825, the “ Drummond Light,” also called 
“ lime or calcium light.” 

Euler , Leonard (1707-83), an eminent Swiss mathe¬ 
matician. In 1733, he accepted the professorship of 
mathematics at St. Petersburg; in 1741, Frederick the 
Great, appointed.him professor of natural sciences in 
the newly erected Academy of Sciences at Berlin; in 
1766, he returned to St. Petersburg, where he remained 
to the end of his life. The last fifteen years of his 
active life he was blind, but that did not prevent him 
from still publishing several important works. His valu¬ 
able “Treatise on Dioptrics” (Dioptrica), in three 
volumes (1769-71), was dictated by him when blind. 


OPTICIANS, SCIENTISTS AND INVENTORS. 209 

He was a great admirer of Newton, whose marvelous 
achievements he investigated most critically; this led 
him sometimes to correct Newton, as we see in regard to 
achromatism. 

Fahrenheit , G. D. (1686-1736). 4 He was the first who 
used mercury in thermometers (1714), instead of col¬ 
ored alcohol. He determined the zero-point by mixing 
salt with chopped ice, contrary to Reaumur, who put 
the zero at the freezing-point of water. He also invented 
the first practical areometer, to measure the specific gravi¬ 
ty in fluids, and the first thermo-barometer. 

Faraday , Michael, ( 1791-1867), one of the most dis¬ 
tinguished chemists and natural philosophers of the pres¬ 
ent century. He was born near London, and was early 
apprenticed to a bookbinder; he devoted his leisure time 
to reading books and making experiments with an elec¬ 
tric machine of his own construction. Humphrey Davy, 
professor of chemistry at the Royal Institute, engaged 
him (1812) as his assistant, and here he first showed 
some of that extraordinary power and fertility which 
have rendered his name familiar to everyone acquainted 
with physics. In 1827, he-was appointed a regular pro¬ 
fessor of chemistry. His greatest work published is the 
series of “ Experimental Researches in Electricity,” 
which comprise all the investigations and discoveries 
made by him during the last forty years of his active life. 
From 1825-29, in conjunction with Sir John Herschel, he 
tried to improve the manufacture of glass for optical 
purposes. Practically considered, this investigation was 
a failure, but the “heavy glass” they produced led af¬ 
terward to two of his greatest discoveries: the “ mag¬ 
netization of light,” and the “diamagnetism.” 

Fasoldt , Charles, (1814-89), of Albany, N.Y., was 
a chronometer-maker by profession, but devoted himself 
in his later years to optical science. He was a mechanic 
of marvelous ingenuity and wonderful exactness and 
skill; his greatest invention is a machine for micrometric 
rulings, of a peculiar construction. His latest rulings 


210 


HAND-BOOK FOR OPTICIANS. 


were so fine that the strongest microscopes could not re¬ 
solve them, till he invented the 44 vertical illuminator ”, 
by which some expert microscopists succeeded in the res¬ 
olution of 230,000 lines to an inch; but his machine is 
capable of ruling one million lines to an inch. His rul¬ 
ings are the best test to determine the strength of micro¬ 
scopes. 

Fitz, Henry, ( 1808-63), a skillful telescope-maker; 
was a printer, but afterwards learned the trade of lock¬ 
smith. In 1835, he made his first telescope, and in 
1845, he exhibited an instrument that brought him into 
favorable notice of eminent astronomers. He made tele¬ 
scopes for the University of Ann Arbor, Mich., for the 
Washington University of St. Louis, for the Dudley 
University at Albany, etc. His largest telescope had an 
aperture of thirteen inches. He was an optician entirely 
by his own tuition. 

\ 

Franklin , Benjamin, (1706-90), an illustrious Ameri¬ 
can statesman, and one of the founders of our Republic; 
invented the lightning-rod, and the bi-focal spectacles, 
which were named after him. 

Fraunhofer , Joseph, ( 1787-1826), the greatest Ger¬ 
man optician, instructed himself in lens grinding, was 
employed, 1806, as a working optician in the establish¬ 
ment of Reichenbach & Utzschneider. While there, he 
acquired considerable wealth through his inventions, and 
became sole proprietor of the establishment in 1819. 
One of his first inventions was a machine for grinding 
and polishing mathematically uniform spherical and par¬ 
abolic surfaces; he also was the first who succeeded in 
polishing lenses and mirrors without altering their curva¬ 
ture. He invented a new heliometer and a circular stage- 
micrometer for microscopes; his improved crown and 
flint glass superior to any English, enabled him to manu¬ 
facture his renowned achromatic microscopes and tele¬ 
scopes. But that which rendered his name celebrated 
throughout the scientific world is his discovery of the 
lines in the solar spectrum ( 1815), called Fraunhofer’s 


OPTICIANS, SCIENTISTS AND INVENTORS. 211 

lines, which were first noticed by Wollaston in 1802, 
but F. published an illustrated map of fully 570 of them, 
assisted in his discovery by large prisms he had made of 
his clear flint glass. His tombstone bears this inscrip¬ 
tion: Approximavit sidera , (he drew the stars nearer). 

Fresnel , A. J., ( 1788-1827), celebrated French physi¬ 
cist and inventor. His researches on the aberration, 
diffraction and polarization of light, completely over¬ 
threw Newton’s Emission Theory, and proved the cor¬ 
rectness of Thomas Young’s defense of the Undulatory 
Theory of Light. His work on the “ diffraction of 
light ” was crowned by the Academy of Sciences in 1819. 
He also considerably improved the system of illumina¬ 
tion for lighthouses. 

Galezowsky , Xavier, born 1833, in Poland, studied 
medicine at St. Petersburg; went to Paris in 1858, became 
the assistant of the celebrated oculist Desmarres, and 
subsequently erected a clinic for eye-patients. He in¬ 
vented the trial-frame with the half circle attached, 
divided into degrees, for the determination of the faulty 
meridian in astigmatic eyes. 

Galilei, Galileo, (1564-1642), the creator of experi¬ 
mental science, was born at Pisa, Italy; studied first 
medicine and philosophy, then mathematics. He util¬ 
ized the pendulum in the construction of a clock for 
astronomical purposes, and invented a hydrostatic bal¬ 
ance by which the specific gravity of solid bodies might 
be ascertained with the nicest accuracy. He also dis¬ 
covered the laws of motion, i. e. that all falling bodies 
of the same specific gravity, great or small, descend 
with equal velocity. Among other discoveries may be 
noticed a certain species of thermometer, a proportional 
compass or sector, also the construction* of a refracting 
telescope for astronomical investigations and of a micro¬ 
scope. By means of his telescope he commenced his 
astronomical researches; he found that the moon was not 
self-luminous, but owed her illumination to reflection, 
and pronounced the milky way a track of countless sep- 


212 HAND-BOOK FOR OPTICIANS. 

arate stars. In 1610, he discovered the four satellites 
of Jupiter; he also was the first to note movable spots on 
the disk of the sun, from which he inferred the rotation 
of that orb. He soon openly advocated the Copernican 
system, and was in consequence denounced as a pro¬ 
pounder of heretical views, and summoned to appear 
before the Inquisition. The persecutions to which he 
was subjected b}^ this “ sacred court,” lasted with short 
intervals almost twenty years. The wearisome trials and 
his incarcerations from time to time only ceased with his 
retractation. On June 22d, 1633, Galilei, at the age of 
seventy years, on his knees, and clad only in a shirt of 
sackcloth, was forced (by torture?) to pronounce in the 
presence of his judges and a large assembly of prelates, 
a most humiliating formula of abjuration. It has been 
asserted that he added in a whisper, “E pur si muove,” 
(still it does move), meaning the earth. 

Galvani , Luigi (1737-98), an Italian physicist and 
celebrated anatomist; discovered accidentally the electric 
current produced by connecting two metals of different 
density, called after him “ Galvanism.” All electro¬ 
plating is based on this discovery. 

Gascoigne , W. (1612-44), an English astronomer and 
mechanic; improved the grinding of lenses. He w T as 
the original inventor of the wire micrometer, of its 
application to the telescope, and of the application of 
the telescope to the quadrant. 

Godfrey , Thomas, born in Philadelphia; worked as a 
glazier in his native city, and studied mathematics with 
great energy; he even learned Latin in order to read 
mathematical works in that language. In 1730, he 
communicated an improvement he had made in the quad¬ 
rant, and the invention was laid before the Royal Society 
in London. In the mean time, John Hadley had made 
a very similar invention, and each of them was awarded 
the prize of £200. Godfrey died in Philadelphia in 1749. 

Graefe , Albrecht von (1828-70), the most celebrated 
German oculist; studied medicine at Berlin, Vienna and 


OPTICIANS, SCIENTISTS AND INVENTORS. 


213 


Paris; established 1850, at Berlin, a clinic for eye- 
patients, and was in 1856 elected professor of ophthal¬ 
mology. He is the founder of modern Ophthalmology, 
greatly assisted by the invention of Helmholtz’s ophthal¬ 
moscope, which received in Graefe’s hands its highest 
recognition. 

Graham , George (1675-1751), an English watchmaker 
and optician; invented the compensated mercury pendu¬ 
lum, also the cylinder escapement, and the dead-beat 
escapement for clocks. He constructed the sextor with 
which Bradley, at Oxford, detected the aberration of 
light, and executed a great mural-arc for professor 
Halley, a celebrated English astronomer (1656-1742), 
at the Observatory of Greenwich, who calculated the 
course of twenty-four comets; one of them bears his 
name. 

Gregory , James (1638-75), a Scotch mathematician, 
invented at the age of twenty-four, the reflecting tele¬ 
scope known by his name. When he went to London 
with the view to the construction of his telescope, he 
found the opticians he employed wanting in the skill 
necessary for grinding the metal of the object-speculum 
into a conic section to correct spherical aberration; 
therefore, he abandoned the manufacturing plan, and 
devoted himself to the study of astronomy. [See James 
Short.] 

Grimaldi , E. M. [1613-63], an Italian jesuit and 
great mathematician; his valuable work on light was 
published two years after his death. He was the first 
who described the “phenomena of diffraction,” or the 
bending of waves of light around the edges of opaque 
bodies. Newton could not explain this phenomenon by 
his emission theory, but Young and Fresnel demonstrated 
its correctness by the wave theory on the “ principle of 
interference.” 

Guerike , Otto von [1602-86], the ingenious burgo¬ 
master of Magdeburg, is renowned as the inventor of the 
air-pump and as the originator of many experiments in 


214 


HAND-BOOK FOR OPTICIANS. 


natural philosophy. He introduced his invention by con¬ 
structing two hollow hemispheres of brass, which fitted 
air-tight upon each other, and which could not be pulled 
asunder, after he had exhausted the air out of them, but 
by the application of great force. They are called the 
Magdeburg Hemispheres, and are still used in experi¬ 
mental physics to show the immense atmospherical press¬ 
ure upon a vacuum. 

Guinand , Francois, (1745-1825), a Swiss watchmaker 
and optician, was the son of a carpenter, and first em¬ 
ployed by the celebrated mechanic Jaquet-Droz, to make 
wooden cases for clocks, and later on, metal cases for 
watches. His employer had a fine English reflector 
which G. so perfectly imitated that it was difficult to de¬ 
cide which of the two was better. Droz being aware of 
the talent of his workman, instructed him in the science 
of optics, in the manufacture of spectacle glasses, and 
in the construction of achromatic lenses. He now stud¬ 
ied chemistry, and commenced to perfect the fabrication 
of lenses for telescopes. Some of these coming under 
the observation of Fraunhofer, he engaged his services. 
The phenomenal improvements of achromatic instru¬ 
ments is due to the combined efforts of these two skillful 
men. 

Hadley , John, [died 1744], an English astronomer, 
greatly improved the quadrant by turning it into a sex¬ 
tant, about the year 1731. [See Godfrey.] 

Harrison, John, [1693-1776], a celebrated English 
watchmaker, learned from his father the trade of "car¬ 
penter, made several clocks of wood with a newly con¬ 
structed pendulum [1726]. He then commenced to 
make watches of metal with improvements of his own 
invention, until 1736, he produced a marine chronome¬ 
ter for which he received the medal. Captain Byron 
took one of his chronometers along on the “voyage 
round the world,” 1764-66, and it proved to be a perfect 
time-piece; he, therefore, claimed the prize of £20,000, 
which the government had offered for the best chronome¬ 
ter. 


i 


OPTICIANS, SCIENTISTS AND INVENTORS. 215 

Helmholtz , H. L. F., born 1821 at Potsdam, Prussia, 
is at present the most famed physicist in Germany. He 
studied medicine at Berlin, and was, 1848, appointed 
professor of anatomy. The next year he went to 
Konigsberg as professor of physiology, where he stayed 
till 1855, when he was called to Bonn. In 1858 he 
accepted the professorship of physiology at the Univer¬ 
sity of Heidelberg. His principal publications are ‘‘Con¬ 
servation of Force,’’ 1847; “ Handbook of Physiological 
Optics,” 1856; “ Theory of the Impressions of Sound,” 
1862; “Popular Scientific Lectures,” 1865-71; besides 
many other valuable scientific papers. He is the inventor 
of the Ophthalmoscope, an instrument which has totally 
revolutionized the science of Ophthalmology, and which 
is at present indispensable to any oculist. He is the 
survivor of the illustrious triumvirate “ Graefe , Helm¬ 
holtz , Bonders ,” who raised Ophthalmology to an exact 
science. Their names will be remembered as long as a 
grateful posterity will cherish the achievements of great 
men. 

Herschel , F. William [1738-1822], born in Hannover, 
educated a musician, emigrated 1757 to England, devoted 
most of his time to the study of astronomy; but being 
too poor to buy a telescope, he built, 1774, a reflector 
five feet long. With the assistance of his brother, who 
was a skillful mechanic, he constructed, 1785, a tele¬ 
scope of forty feet in length, which was the most power¬ 
ful instrument at that time, and with which he made 
many discoveries, He discovered the planet Uranus, 
and some of his moons, also two moons of Saturn. 

Herschel, Sir John, [1792-1871], followed in the 
footsteps of his celebrated father, whom he greatly ex¬ 
ceeded in profound mathematical science, as well as in 
the long list of his astronomical researches and discover¬ 
ies. In 1830 he published a treatise “ On the Theory of 
Light,” comprising his investigations in the optical 
department, which he had made in conjunction with Far¬ 
aday. In 1838, Queen Victoria created him a baronet. 
He was an indefatigable explorer, and the most success¬ 
ful astronomer of this century. 


216 


HAND-BOOK FOR OPTICIANS. 


Heurteloup , Nic., [1750-1812], celebrated surgeon in 
the French army; invented the artificial leech. 

Hipparchus , considered the founder of the science of 
astronomy; lived about 150 years before Christ, and was 
born at Nicsea, Bithynia [Minor Asia]. Of his life 
nothing is known, and of his writings only one book has 
been left to us; but Ptolemy tells us of his great discover¬ 
ies, and refers to him in many cases as an authority. 

Hooke , Robert, [1635-1703], watchmaker at London; 
invented, 1658, the balance spring [hairspring], also the 
anchor-pallets for clocks, and a sliding-weight to the 
pendulum, to adjust the center of gravity with greater 
precision. 

Huyghens , Christian, [1629-95], of The Hague, Hol¬ 
land; one of the greatest discoverers in mathematics, 
physics and astronomy. He discovered the law of double 
refraction in crystals with one axis, opposed the “ emis¬ 
sion theory” against Newton, and founded the “undula- 
tory theory.” He improved telescopes, ground and 
polished the lenses himself, and introduced the connec¬ 
tion of the pendulum with clock-work. He also discov¬ 
ered the ring and the fourth satellite of Saturn. [See 
Galilei]. 

Jaeger , Edward, son of the celebrated Frederick Jae¬ 
ger, is professor of ophthalmology at the University of 
Vienna, Austria. In 1854, he published his test-types, 
ranging from the finest to very large letters, in different 
languages. Among his many excellent publications, the 
most famous is his “Atlas of Ophthalmology”, the origi¬ 
nal drawings of which were afterwards purchased by 
Dr. Norris of Philadelphia for about $2000. 

Johnston , J. M., born 1844 in Western New York, an 
able optical writer; was educated for the Church, but 
entered, 1880, the Johnston Optical Company. He 
issued 1886 the “Eye-Echo,” the first journal in America 
devoted exclusively to optics, and as a continuation of 


OPTICIANS, SCIENTISTS AND INVENTORS. 


217 


the former, since 1891, the “Eye-Light.” In 1892, he 
published a valuable work, “Eye Studies, a series of les¬ 
sons on vision and visual tests”. 

Kepler , John, [1571-1630], a German mathematician 
and astronomer of great reputation; was one of the 
founders of modern astronomy. His three laws [Regu- 
lae Kepleri] of the elliptical orbits of the planets were 
afterwards accepted by Newton, and are still in use. lie 
invented the astronomical telescope in which the object¬ 
ive and ocular lenses were both convex. He was the 
first who explained the true theory of vision. 

Kircher , Athanasius, (1601-80), a very learned Jesuit; 
was born in Germany, but lived mostly in France and 
Italy. He invented the Magic Lantern, and constructed 
a powerful burning-mirror with which he experimented 
on the Island of Malta; it is known by the name Mal- 
tesian Mirror. 

Kirchhoff , G. R., [1824-87], celebrated German phy¬ 
sicist, born at Konigsberg, Prussia; studied mathematics 
and physics, went 1847 to Berlin as professor of physics, 
1850 to Breslau, 1854 to Heidelberg, and 1875 again to 
Berlin. His scientific researches were mostly directed 
to electricity, galvanism, and to the peculiar properties 
of bodies and gases. His investigations of the Fraun¬ 
hofer’s lines, which he made in conjunction with Bunsen, 
led them to the discovery of “spectrum analysis.” 

Knapp , H., celebrated oculist of Germany and Amer¬ 
ica; was born 1832 in Nassau, Germany; studied for 
nine years medicine at Munich, Berlin, Leipzig, Vienna, 
Paris, London and other celebrated Universities. He 
was lecturer and later professor of ophthalmology in 
Heidelberg, but resigned in 1868, and settled in New 
York City^ Here he published the Archives of Ophthal¬ 
mology and Otology, and founded the N. Y. Ophthalmic 
and Aural Institute. He was for several years profes¬ 
sor of ophthalmology at the Medical College of the Uni¬ 
versity of N. Y., and is at present professor of ophthal¬ 
mology at the College of Physiology and Surgery at 


218 


HAND-BOOK FOR OPTICIANS. 


New York. He is regarded as an authority in medical 
circles, in America as well as in Europe. In 1873, he 
introduced some very valuable improvements in the 
Ophthalmoscope. 

Lieberkuhn , J. N. [1711-65], physician at Berlin; 
invented, 1738, the solar microscope. 

Lippershey , John, the inventor of spyglasses, about 
the year 1600, was born in Wesel, Germany; his real 
name was Hans Lippersheim. He established himself as 
an optician at Middleburg, Holland. It is told that one 
day his son was playing with old spectacle lenses, and 
accidentally put a convex lens at one end of a hollow 
tube and a concave one at the other end; then called the 
attention of his father to the strange phenomenon, that 
distant objects seemed to be so near-by, that he fancied 
he almost could touch them. 

Littrow , J. J., [1781-1840], studied at Prague, was 
engaged at different universities as professor of mathe¬ 
matics and astronomy, until in 1819, he became director 
of the Observatory at Vienna. Some of his theoretical 
publications induced the optician Ploessl to construct the 
dialytic telescope. His most popular publications are: 
“The Wonders of the Heavens,” and “Maps of the 
Starred Heavens ’ ’. 

Malpighi, Marcello, [1628-94], an Italian anatomist; 
was the first to employ the simple microscope to investi¬ 
gate the anatomical structure of plants and living animals; 
thus he discovered the capillary circulation of the blood 
from the arteries to the veins. Various parts of the 
epidermis, spleen and kidneys still bear his name. 

Malus , E. L., [1775-1812], a French physicist, was 
educated at the school of military engineers; was a great 
mathematician, but took a fancy to the study of the 
mathematical theory of optics'. For the greater portion 
of his short life he was attached to the French army, and 
took part in the adventurous expedition of Bonaparte 


OPTICIANS, SCIENTISTS AND INVENTORS. 219 

[Napoleon I] to Egypt. In 1801, he returned to Paris, 
and although his health was broken down, his spirit was 
yet in the prime of life. In 1808, the French ‘‘Institute 
of Sciences” offered a prize for the best essay on double 
refraction in crystals. Malus competed for the prize, 
and in the course of his experiments discovered the phe¬ 
nomenon known as the polarization of light. He ad¬ 
vanced the theory “that particles of light have poles, 
and that on entering a doubly-refracting crystal, some of 
the particles forming one of the rays may be so arranged 
as to be transmitted through it, while the particles which 
should have formed the other ray may be so arranged as 
to prevent the transmission in certain directions.” 0 This 
discovery introduced a new diversion of physical optics. 
In 1810, he published his “ Treatise on Optics,” and his 
“ Theory of the double refraction of light in crystals”. 

McAllister , John, [1753-1829], born in Scotland, emi¬ 
grated to America in 1774, and started, 1796, an optical 
business in Philadelphia. John McAllister jr. [1786- 
1877], a graduate from the University of Pennsylvania, 
associated with his father in 1811, and laid the foundation 
of an extensive business. The war of 1812 stopped the 
importation of spectacles, and compelled them to manu¬ 
facture all gold and silver spectacles themselves. In 
1836, \\ alter B. Dick and Jas. W. Queen became part¬ 
ners, till 1853; the firm, McAllister & Co., was then 
continued by him and his son, W. Y. [born 1812], until 
1865, when the father retired. In 1882, W. Y. McAl¬ 
lister took his son, W. M. [born 1843], as partner; an¬ 
other son, F. W. [born 1853], started an optical business 
in Baltimore, 1879, and is the inventor of an improved 
nose piece.—This remarkable “ family of opticians ” will 
soon celebrate its centennial. 

Merz & M&hler , the skillful successors of Fraunhofer, 
at Munich; turned out many astronomical telescopes] 
among them the famous refractor of the Pulkowa Ob¬ 
servatory in Russia, also that of the Harvard University 
in the U. S.; both instruments contain object lenses of 
fifteen inches aperture. 


15 


220 


HAND-BOOK FOR OPTICIANS. 


Mudge, Thos., [1710-94], an English mechanic; was 
an apprentice of the celebrated Graham, and became 
the most skillful watchmaker in Europe. The English 
government paid him for the superiority of his chronome¬ 
ters the prize of twenty-five hundred pounds sterling. 
He invented the lever escapement. 

Newton , Sir Isaac, [1642-1727], the most remarkable 
mathematician and natural philosopher of his age, was 
the founder of modern mathematical physics and physi¬ 
cal astronomy. In 1665 he discovered the law of uni¬ 
versal gravitation; he then studied the nature of light, 
and detected by means of prisms the composition of 
white light, which led him to the grinding of lenses, and 
to the construction of reflecting telescopes. In 1704 he 
published his “Optics, or a treatise on the reflections, 
refractions, inflections and colors of light;” and in 1713 
his “ Principia.” He was the founder of the “emis¬ 
sion theory.” 

Nicholson , W. [1753-1815], English physician and 
chemist; invented the areometer or hydrostatic balance, 
that bears his name. He published about twenty sci¬ 
entific works mostly on chemistry. 

Nicol, W., [1768-1851], a lapidary at Edinburgh; in¬ 
vented the polarizing prism of Iceland spar, which totally 
reflects the ordinary ray, whilst the extraordinary passes 
through, and which bears his name. His skill as a work¬ 
ing lapidary was very great; he executed a number of 
lenses of precious stones, especially of garnet, which 
lenses he preferred to the achromatic microscopes of his 
time. 

Porta , Battista, (1540-1615), an Italian astronomer, 
founded an academy in Naples to which no one was ad¬ 
mitted unless he had made some discovery in natural 
philosophy. He was accused of magic and compelled 
by the pope to dissolve his academy. He wrote many 
volumes on natural magic, geometry, optics, etc.; also in¬ 
vented the camera obscura , and demonstrated that visual 


OPTICIANS, SCIENTISTS AND INVENTORS. 221 

perception is not effected by rays emanating from the 
eye, but by rays reflected from objects. (See Alhazen). 

Prentice , James, (1812-88), eminent American opti¬ 
cian; was born in London, served an apprenticeship of 
seven years with Elliott & Son, opticians and mathemat¬ 
ical instrument manufacturers in London. He emigrated 
to America in 1842, and almost immediately secured the 
government patronage of the U. S., which he continued 
to supply with instruments until the beginning of the 
war, 1860. The superior excellence of his instruments 
gained for him a far-reaching reputation among archi¬ 
tects and engineers. He received nine medals and four 
diplomas of honor between the years 1842 and 1860. 
After the opening of the war, he devoted his entire at¬ 
tention to the store which he had just previously opened. 
In 1867, he invented and patented the “anatomical eye¬ 
glass,” since universally known as the Prentice eye glass , 
which was the beginning of improvements in eyeglass- 
frames; numerous patents by others soon following it. 

Prentice , Chas. F., son of James Prentice, was born in 
Brooklyn, 1854; attended the Royal Polytechnicum at 
Carlsruhe, Baden, from 1871-74. It was his father’s 
desire that he should give particular attention to mechan¬ 
ics, physics and mathematics, in order to become a thor¬ 
ough optician, as the father justly anticipated that the 
great development of the optical trade would lay greater 
claim to the ability of the future optician. After his re¬ 
turn to America, he temporarily accepted the position as 
mechanical draftsman at the shipyard of John Roach in 
N. Y.; but in 1878 he entered his father’s business, where 
he became a partner in 1883, and since 1888 its proprie¬ 
tor. His former theoretical studies soon made him the 
foremost optical writer in America. In 1886, he published 
his valuable “Treatise on Ophthalmic Lenses;” in 1888, 
his mathematical and most scientific “Dioptric Formulae 
for Combined Cylindrical Lenses,” and in 1890, his 
“Metric System of Measuring Prisms.” He simul¬ 
taneously invented the “Prismometer,” to determine the 
refractive properties of prisms by their deviation; a new, 


222 


HAND-BOOK FOR OPTICIANS. 


simple and most ingenious method, which enables ocu¬ 
lists and opticians to experiment with prisms in a more 
scientific manner than ever before. 

Ptolemy , Claudius, an Egyptian astronomer, flour¬ 
ished at Alexandria in the middle of the second century 
after Christ. He wrote the “Syntaxis Mathematica,” 
which is a representation of the science of astronomy of 
that time, based partly on his own researches, partly on 
those of Hipparchus. As it is the only authority we 
have for the views of astronomy entertained by the an¬ 
cients, and as it formed the foundation of all astronom¬ 
ical science down to the time of Copernicus, the book is 
consequently of the greatest interest. 

Queen, James W., [1812-90], an American optician; 
learned his trade at the establishment of John McAllister 
at Philadelphia, in which he afterwards became a part¬ 
ner. In 1853, he commenced business for himself and 
gradually built up the largest scientific optical house in 
America. 

Pamage , optician at Aberdeen; constructed, 1820, a 
Newtonian telescope at the Royal Observatory of Green¬ 
wich. The speculum has a focal length of twenty- 
five feet, and a diameter of fifteen inches. It was at 
that time the largest instrument in Europe. 

Pamsden , Jesse, (1735-1800), an English optician of 
rare skill; was a dry-goods clerk; learned engraving on 
copper; had to engrave many illustrations of optical in¬ 
struments, which induced him to learn the trade with 
John Dollond. Already in 1763, his instruments had a 
great reputation. He made many improvements and in¬ 
ventions, of which his “dividing machine” is the most 
important. He constructed some “mural circles,” one 
of five feet diameter for Palermo, Italy, and one of eight 
feet for the Observatory at Dublin. The error of one of 
his quadrants (at Padua) was only two seconds. 

Reaumur , R. A. F., (1683-1757), celebrated French 
physicist; divided the scale of the thermometer from the 


OPTICIANS, SCIENTISTS AND INVENTORS. 223 

freezing to the boiling point of water, into eighty de¬ 
grees. His thermometers were filled with colored alco¬ 
hol, which is preferable in great cold, as mercury will 
freeze at a temperature of forty degrees below zero, 
while the severest cold has never yet frozen pure alcohol. 

Reichenbcich , Geo., [1772-1826], became with Fraun¬ 
hofer the ornament of the “Mechanical and Optical In¬ 
stitute of Bavaria” at Munich. His astronomical instru¬ 
ments, meridian circles, transit instruments, equatorials, 
heliometers, etc., made an epoch in “observing astron¬ 
omy.” 

Riddell , J. L., [1807-67], of New Orleans, was pro¬ 
fessor of botany and chemistry at the University of Loui¬ 
siana, 1836-65. He was the original inventor of the 
binocular microscope, 1851, which was afterwards manu¬ 
factured and introduced by J. W. Stephenson of Lon¬ 
don. He also constructed an achromatic binocular mag¬ 
nifier in the form of spectacles, leaving both hands of the 
operator free for manipulation, and which is still in pos¬ 
session of the well-known oculist, Dr. Cornelius Beard, 
forfnerly of New Orleans, now in Boston. 

Rittenliouse, David, [1732-96], an American mathe¬ 
matician; made the first telescope ever constructed in 
America. He learned clock-making, and established 
himself, 1751, in Norriston, near Norristown, Pa., as a 
clock and mathematical-instrument-maker. His days 
were spent in folloAving his trade, and his nights were 
given to study. His orrery exhibits almost every motion 
ill the astronomical world; it was bought by the Univer¬ 
sity of Pennsylvania for £400. In 1770, he removed his 
business to Philadelphia. His scientific instruments dis¬ 
played unusual mechanical and mathematical genius. 

Rochon , Alexis, (1741-1817), a French astronomer; 
was first abbot of a convent, but quitted the church, and 
studied optics and astronomy. In 1777, he constructed 
a micrometer of rock crystal-to measure small angles. 
He made several scientific expeditions to French colonies 
in Africa and East Indies, and found in Madagascar the 


224 


HAND-BOOK FOR OPTICIANS. 


finest rock crystal which he ground into lenses; but de¬ 
clared them, afterwards, to be unfit for spectacles on ac¬ 
count of their double refraction. 

JRoemer , Olans, (1644-1710), Danish astronomer; dis¬ 
covered the velocity of light by the eclipses of the first 
moon of Jupiter. The other three moons were not so 
favorable for this observation, as their mutual attraction 
makes their motion more complicated, and puzzled the 
astronomers, till Newton published his theory of univer¬ 
sal gravitation, which solved the mystery. 

liosse, Lord W. P., (1800-67), the distinguished con¬ 
structor of the largest reflecting telescope. In 1845, he 
built his great reflector, which up to the present day has 
remained without a rival. It has a focal length of fifty- 
four feet, and the tube is about seven feet in diameter. 

/Saxton, Joseph, (1799-1873), a skillful American me¬ 
chanic; was apprenticed to a watchmaker, went 1817 to 
Philadelphia, where he worked at his trade, but devoted 
much time to drawing and engraving. He constructed 
an astronomical clock with an escapement on a new plan. 
In 1828, he went to England, where he made many in¬ 
genious mechanical toys, and exhibited, 1833, a magneto¬ 
electric machine, with which he produced a brilliant 
electric spark, decomposed water, and exhibited the 
electric light between charcoal points. In 1837, he re¬ 
turned to Philadelphia, and accepted the position of con¬ 
structor of the standard weighing apparatus of the U. S. 
mint. He invented the medal-ruler, the fountain pen, 
and other useful machines and appliances. 

/Short , James, (1710-68), born in Edinburgh; con¬ 
structed about 1732, some telescopes for his own amuse¬ 
ment. In his first telescopes the specula were of glass, 
as suggested by Gregory, but he afterwards used metal¬ 
lic specula only, and succeeded in giving to them true 
parabolic and elliptic surfaces. He then adopted tele- 
scope-making as his profession, and went to London. 
All his telescopes were of the “Gregorian” form, and 


OPTICIANS, SCIENTISTS AND INVENTORS. 


225 


some of them have retained even to the present day their 
original high polish and sharp definition. 

Snell, Willebrord, (1591-1626), a Dutch mathemati¬ 
cian; discovered the “law of the sines,” i. e. that the 
sines of the angles of incidence and refraction are con¬ 
stant for the same medium.—Kepler tried to find this 
law, but did not succeed. 

Snellen , H., was the pupil and assistant of Donders, 
and since 1888, is his successor as attending oculist to 
the Netherland Eye Hospital; is also professor of Oph¬ 
thalmology at the University of Utrecht, Holland. In 
1868, he published his test-types which virtually solve 
the problem of registering vision. 

f 

Spencer , Chas. A., (1813-81), born in Lennox, N, Y., 
is considered the pioneer of scientific optics in this coun¬ 
try. He received a classical education at different col¬ 
leges, but his attention was soon drawn to more practi¬ 
cal study and experiment by himself. In 1831, he set¬ 
tled in Canastota, N. Y., as a manufacturer of telescopes 
and microscopes. He issued a descriptive catalogue, 
“Optical, Philosophical, Mathematical, Chemical and 
other Instruments and Apparatus,” which contained a 
list of prices of various sized reflecting telescopes, 
chiefly of the Newtonian and Gregorian construction; 
only a few small achromatic telescopes and microscopes 
were mentioned. The instruments of this character then 
in the country were imported ones, and probably the 
whole number of achromatic microscopes was less than 
a dozen. The fame of Powell, Ross, Chevalier and 
Amici instigated his ambition to surpass them. He 
commenced to construct objectives of a considerable larger 
angle of aperture than in the European instruments. He 
first used some of Guinand’s improved flint glass, but 
afterwards made extended and costly experiments in the 
attempt to produce a glass of higher dispersive and refrac¬ 
tive power, and was to a certain extent successful; although 
his chief success was his skill in giving his lenses such curves 
which nicely balanced the aberrations. Every microscope 


226 


HAND-BOOK FOR OPTICIANS. 


was accompanied by some fine object-slides to show its 
power, and which gradually became so fine that the English 
microscopists could not resolve them with their instru¬ 
ments. The first microscope that attracted attention was 
made for Dr. Gilman in 1847. Spencer’s name became 
at once famous, A great deal of his success, was due to 
the encouragement he had from American scientists, such 
as Prof. J. W. Baily, of West-Point, Dr. John Torrey, 
Dr. Goring, Dr. Gilman and Dr. John Frey, of N. Y., 
Dr. C. A. Beck and Paul Goddard, of Philadelphia, 
Thomas Cole, of Salem, Mass., and others, who purposely 
hunted for finer and finer tests in order that Spencer 
should resolve them, which he did. It is delightful to 
read to-day the few letters which remain of the volu¬ 
minous correspondence which was carried on between 
those early microscopists, when the new powers of the 
microscope were just being unfolded, and a whole world 
of original investigations, full of marvels and wonders, 
was opened before them. An almost boyish enthusiasm 
appears to have animated them, as is apparent in their 
familiar correspondence. Spencer was now fairly a rival 
of the best foreign artists, and was acknowledged by them 
as such. Instead of following up this special branch, he 
diverted his attention to the study of astronomy which 
was particularly fascinating to him, and his fondness for 
the telescope and telescopic pursuits never diminished, 
though he found in the development of the microscopical 
objective a more promising field for his genius. About 
the year 1854, he formed a partnership with A. K. Eaton, 
and in addition to the microscopical work, they completed 
various achromatic telescopes, among them the large 
Equatorial for Hamilton College, having an object glass 
of 13£ inches in diameter, and a focal length of 16 feet. 
This was then the largest telescope in this country, 
and in its performance it compared favorably with 
the best Munich instruments; its price was $10,000. 
In 1856, they entered into a contract with the Trustees 
of the Dudley Observatory at Albany, to construct a 
magnificent heliometer, for the sum of $14,500. It was 
agreed that Spencer should visit the principal workshops 
of Europe and the celebrated Observatories, in order that 


OPTICIANS, SCIENTISTS AND INVENTORS. 


227 


the instrument might surpass anything hitherto made. 
While he was absent, his optical department was managed 
by R. B. Tolies, who had been for some years his pupil. 
At Spencer’s return, after an absence of six months, 
a bitter controversy arose between some of the 
Trustees and Dr. B. A. Gould, the director of the 
Observatory, which suspended the work for years ; in 
fact, the heliometer was never built. The partnership 
between Spencer and Eaton was dissolved after a few 
years; he, with the aid of his sons, still carried on the 
business, until the year 1873. In the fall of this year 
occurred the disastrous fire at Canastota, which destroyed 
nearly every shop in the village, and very nearly ruined 
Spencer. The fire commenced at night in a building 
opposite his shop, and lasted all night. So rapidly was 
the spread of the fire, that he nearly lost all his tools and 
machinery, the accumulation of many years of toil and 
skill, and a large amount of finished and unfinished work. 
Only the building was insured. Crippled, but not wholly 
disheartened, he and his sons, with what they had saved 
from destruction, commenced anew in a little barn for a 
workshop. In 1875, they moved to Geneva, N. Y., and 
for two years were connected with the Geneva Optical 
Works. From 1877, the business was conducted under 
the name of C. A. Spencer & Sons. During this period 
they received, at the Paris Exposition, the highest award, 
a beautiful large gold medal, for excellence of their 
microscopical objectives. In 1880, his son, Herbert 
Spencer, commenced business in Geneva under his own 
name, while his father remained in the old shop. Al¬ 
ready the evidence of an over-tasked constitution and a 
too long-continued strain upon his mental powers had 
become painfully evident to his friends. He did but 
little work, and spent most of his time in reading, occa¬ 
sionally experimenting with some new combination, but 
always genial and pleasant to such of his old friends as 
from time to time visited him to talk over the past, and 
discuss the future of microscopy and science generally. 
He died, after a confinement to his room of three weeks, 
on Sept. 28th, 1881. — Spencer was a genius in the full 
sense of the word. Life was not to him a contest for the 


228 


HAND-BOOK FOR OPTICIANS. 


possession of wealth; no man was ever more indifferent 
to this than he; if he had been anything else he would 
have accumulated a fortune. He never was satisfied with 
his work, no matter how perfect it was for that time; 
so it happened that very often it cost him much more to 
produce a given piece of work than the pay he received 
for it. Large as were the prices which his acknowledged 
skill enabled him to obtain, — prices which were only 
too willingly paid, so the work could be had at all, — 
yet his life, from boyhood, when with the enthusiasm 
of youth he saw a name and fame opening before him, 
up to the time when, enfeebled by age and disease, he 
entered the eternal rest, was one long struggle with pov¬ 
erty. Not for want of industry. No man was ever 
more industrious, but not in the way of the world. 
There is little money to be made in the tedious testing 
and ‘ 4 touching up ” of that which any one else would 
have called perfect work, but which the artist is unwilling 
to let pass from his hands, except when stern necessity 
compels, so long as he can imagine something better; 
albeit to accomplish this better, may be the work of days, 
weeks, or even months of trial and ardent application. 

Tolies , Robert B., (died 1888 in Boston) ; was the most 
skillful American optician. (To my greatest sorrow I 
could not attain any information about his life, although 
I have written a dozen letters to different parties; three 
to his surviving partner. Indeed, the historian of con¬ 
temporaries travels a hard road). 

Torricelli , E. (1608-47), an Italian philosopher and 
mathematician; invented the barometer, 1643. He was 
also a skilled optician; his single microscopes were of 
great perfection, also his lenses for telescopes. 

Tschirnhausen,'E l .W ., (1651-1708), a German mathe¬ 
matician, physicist and philosopher; erected a large glass 
foundry, principally for the grinding of burning glasses. 
One of his make is still in the Academy of Sciences at 
Paris, which is thirty-three inches in diameter, and 
weighs 160 pounds, but is full of imperfections.. 


OPTICIANS, SCIENTISTS AND INVENTORS. 229 

Tycho Brahe , (1546-1601), celebrated Danish astrono¬ 
mer; enriched the science of astronomy very much, 
partly by his numerous observations, partly by inventing 
new instruments, for instance, the Mural 'Circles. He 
rejected the Copernican system, which in his time was 
not supported by the conclusive evidence we now have 
in its favor. In fact, Tycho’s theory, which made the 
sun move round the earth, and all the other planets round 
the sun, explained all the phenomena then known equally 
well with that of Copernicus. In 1597, he emigrated 
to Germany, and settled at Prague,where Kepler became 
his pupil. 

Tyndall , John, born 1820 in Ireland; studied in his 
leisure hours mathematics and natural sciences, mostly 
by self-tuition. In 1844 he intended to emigrate to 
America, but his subsequent appointment as a surveyor 
on an English railroad changed his mind. In 1848, he 
went to Germany for further study, where he attended 
the celebrated lectures on chemistry by Bunsen, at Mar¬ 
burg. His first scientific publication [in German] was “On 
the Magneto-Optic Properties of Crystals.” On his re- 
tqrn to England, 1852, he made the acquaintance of 
Faraday, whose successor he became a year afterwards, 
as professor of natural philosophy at the Poyal Institu¬ 
tion of London. In 1872, he made a very successful 
lecturing tour in the U. S., treating especially of light, 
heat and sound. His greatest merit as a scientist is his 
marvelous gift of imparting to the great mass a clear 
conception of the most knotty subjects, in a popular way. 

Volta , Allessandro, [1745-1827], celebrated Italian 
physicist; invented 1777, the electrophorus, the electro¬ 
scope and the endiometer. In 1782, he invented the air- 
condenser, [the opposite to Guerike’s airpump].. But 
his greatest achievement was the practical application of 
the invention of Galvani, in constructing the so-called 
Voltaic Pile, the fore-runner of the Electric Lights. 

Wait , James, [1736-1819], an English optician and 
inventor; filled from 1757 to 74, the position of an op- 


230 


HAND-BOOK FOE OPTICIANS. 


tician at the University of Glasgow; then he associated 
himself with Boulton. In 1779, he invented a machine 
for copying letters; but his greatest accomplishment was 
the improvement of steam Engines, and the invention of 
steam condensers. 

Wheatstone , Charles, [1802-75], an English physicist; 
was from early youth a musical instrument maker, which 
led him to investigate the laws of sound. In 1834, he 
was appointed professor of experimental philosophy in 
King’s College of London, when he read an essay enti¬ 
tled “Contributions to the Physiology of Vision.” This 
led to the invention of his stereoscope, which he first ex¬ 
hibited in 1838. He was also the discoverer of important 
practical applications in electrical science. 

Wilson, James; made the first artificial globes manu¬ 
factured in the U. S. He lived at Bradford, Vt., about 
1812. 

Wollaston, W. H., [1766-1828], an English chemist 
and physicist; practiced medicine till 1800, then went to 
London and devoted himself to chemistry and physics. 
He discovered in platinum the metals Palladium and 
Khodium; improved the microscope by introducing 
“doublets;” invented 1807, the “camera lucida,” and 
the “periscopic lenses.” He also invented the “reflect¬ 
ing goniometer,” a valuable instrument to measure accu¬ 
rately small angles of crystals. Plis improvements in 
the construction of galvanic batteries were afterwards 
greatly surpassed by the ingenious inventions of Fara¬ 
day. 

Young, Thomas, (1773-1829), the most profound in¬ 
vestigator since. Newton; studied medicine in London 
and Edinburgh, but devoted himself greatly to the study 
of natural philosophy, to mathematics and optics. As 
early as 1794 he published an “Essay on the act of see¬ 
ing, and on the peculiarities of the crystalline lens,” in 
which lie explained his own case of astigmatism not ob¬ 
served before by others. He is the scientific founder of 


OPTICIANS, SCIENTISTS AND INVENTORS. 231 

the “undulatory theory of light,” and discovered the 
principle of “interference of light.” His main work 
was published 1807, in two volumes, “Course of lectures 
on natural philosophy and the mechanical arts.” His 
contemporaries considered him a “crank,” but subse¬ 
quent discoveries in the same line by the celebrated 
Frenchmen, Fresnel and Arago, and especially the able 
defense of the eminent German scientist, Helmholtz, 
and lately of Tyndall, have restored his fame forever. 

Zentmayer, Joseph, a skillful optician of Philadel¬ 
phia, died 1888. He was a native of Germany, and 
came to America in 1848. He invented several instru¬ 
ments of a scientific nature, for which he was awarded a 
gold medal in 1874. His microscopes are of the finest 
workmanship produced in America. 


CHAPTER XXVIII. 


Miscellanies. 


I. Problems for Inventors .—The following ^suggestions 
are offered to the trade, in order to draw the attention of 
young opticians to these important subjects, and to direct 
their investigation into a proper channel. 

1. Is there a test for the different qualities of glass ? 

2. Can we illustrate by diagram, how two cylinders of 
the same strength, and at right angles, produce a spherical 
lens ? 

3. Is there any .perfectly transparent substance, or can 
it be manufactured, which will absorb all caloric rays, 
transmitting to the eye only the luminous part of the 
light ? (Mica is not perfectly transparent.) 

4. What is the cheapest substitute for steel, so objec¬ 
tionable on account of its getting rusty ? 

5. Can screws be omitted, and clamps and clasps be 
substituted without looking clumsy ? 

6. Can the nose-bridge of spectacles be made move- 
able, up and down, out and in, or be made telescopic, to 
regulate its width according to pupil distance, without 
injuring the appearance or strength ? 

7. Can we construct a contrivance to take the exact 
shape of the nose and the temporal width of the head, as 
a guide to a perfect fit of eyeglass and spectacle frames ? 
The custom of dentists, to use pliable paste in taking 
impressions of the gum is, of course, excluded; but 
how about a device similar to that of a hatter, with small 
movable blocks‘of equal length, to show outside the 
same elevation as inside ? If the rods are placed par¬ 
allel, at right angle to the bar, I think, it can be done, 
and be applied to any face without the least inconvenience 
to the customer. 




MISCELLANIES. 


233 


II. Corundum and Emery .—Corundum in its pure 
state is composed of the oxide of aluminium. It is an 
exceedingly tough, compact mineral, occurring in a great 
variety of colors — blue, red, yellow, to nearly white. 
The pure crystals are translucent, and used as gems. It 
is one of the hardest known minerals, being placed in 
the scale of hardness next to the diamond. This quality 
is the source of its greatest value in the arts. The spe¬ 
cies are divided into three qualities — sapphire, corun¬ 
dum, and emery. 

Sapphire includes the purer kinds of fine colors, trans¬ 
parent or translucent. These stones are used as gems, 
and are known by names indicating their color. The 
following well-known jewels are forms of this mineral: 
ruby, sapphire, oriental emerald, oriental topaz, and 
oriental amethyst. These gems are found chiefly in the 
beds of rivers in Ceylon, though some rubies are brought 
from Syria. The value of these stones was w T ell known 
to the ancients, who used them under various names now 
obsolete. The stone called sapphire by Pliny is now 
known to lapidarists as lapis lazidi. 

The oriental emerald is perhaps the rarest gem known. 
A few specimens have been found among the gold sands 
of the Missouri River, near Benton. But few of these 
jewels are in existence, and these are in the great collec¬ 
tions of Europe. 

Corundum generally means the dull, untransparent 
occurrences of the mineral. They vary in color—blue, 
gray, or brown—but are never clear or capable of being 
cut; it usually occurs in large, rough crystals, or in 
massive cleavages. 

Emery is granular corundum. It is black or grayish- 
black in color, and mixed with grains of magnetite. 
Emery has very much the appearance of fine-grained 
iron ore, and for a long time was considered to be such. 
The texture is variable, some specimens being composed 
of almost impalpable grains, while others are made up of 
large, rough fragments of crystals. 

Until recently the only source of emery was the far 
East, the Island of Naxos, in the Grecian Archipelago, 
containing the chief mines. The emery was shipped 


234 


HAND-BOOK FOR OPTICIANS. 


from the port of Smyrna, and was known to commerce 
as Smyrna emery. 

Emery and corundum are chiefly used in the arts as 
abrading and polishing materials. The mineral is ground, 
and separated by passing through sieves into classes of 
various dimensions, which are then further prepared in 
different ways adapted to the purposes for which they 
are to be used. For the use of jewelers and opticians, 
the fine emery is poured into water containing gum, and 
the coarser particles allowed to settle; the fine, impal¬ 
pable dust remaining suspended in the liquid is then 
collected and used in polishing spectacle lenses, and 
similar articles. The largest amount of emery is used 
by the manufacturers of plate glass, though great quan¬ 
tities come upon the market prepareddn a great many 
different shapes to suit special purposes. One of the 
largest of these industries is the manufacture of emery 
wheels; these are prepared by mixing the powder with 
glue or cement, and subjecting the paste to great press¬ 
ure. Mixed with paper pulp and rolled into sheets, it is 
sold in the form of patent razor strops and knife shar¬ 
peners. Spread out on paper and cloth, it forms an ex¬ 
cellent substitute for sand paper. Recently it has been 
discovered that crystallized corundum, when ground, 
forms a better abrading material than emery, owing to 
the fact that it breaks into sharp edged fragments, while 
emery has rather a rounded form. This discovery was 
followed by the discovery of large deposits of corundum 
and emery in Massachusetts, North Carolina, and 
Georgia. All of these localities are being actively 
worked, and large quantities of American material are 
being put on the market. 

In the near future it is probable that corundum will 
assume a far more prominent place among the useful mine¬ 
rals as the source of the metal aluminiumThe cheap pro¬ 
duction of this metal has long been the object of experi¬ 
ment to metallurgists; and corundum, furnishing the 
purest source from which it can be obtained, will prob¬ 
ably be the most valuable ore. 

Ill . How to separate Lenses .—The two lenses of an 
achromatic object glass are cemented together with 


MISCELLANIES. 


235 


Canada balsam, the volatile part of which passes away, 
after a time, and it frequently happens that air or moist¬ 
ure, taking the place of this, gives an iridescent appear¬ 
ance to the glass and interferes with correct delineation. 
To remedy this fault it becomes necessary to separate and 
clean the two lenses and readjust them, cementing with 
Canada balsam, as before. Hitherto it has been custom¬ 
ary, in order to effect the separation, to apply heat, and 
however carefully this may be done, it sometimes hap¬ 
pens that a lens is thereby cracked. All risk of fracture 
may be avoided by placing the achromatic combination 
in a small quantity of benzole or naphtha (from coal tar) 
within a covered vessel, either of which hydrocarbons 
will, in a day or two, dissolve away or soften the hard¬ 
ened cement without heat. The same liquid will remove 
the last traces of resinous matter. 

IV. The Sharpening of Tools. — Instead of oil, 
which thickens and smears the stone, a mixture of gly¬ 
cerine and spirit is recommended. The proportions of 
the composition vary according to the class of tool to be 
sharpened. One with a relatively large surface is best 
sharpened with a clear fluid, three parts of glycerine be¬ 
ing mixed with one part of spirit. A graver having a 
small cutting surface only requires a small pressure on 
the stone, and in such cases the glycerine should be 
mixed with only two or three drops of spirit. 

V. A Dead Black Paint for Optical Instruments. 
—Take two grains of lamp black, put it into any smooth, 
shallow dish, such as a saucer or small plate, add a little 
gold size and thoroughly mix the tWo together. Just 
enough gold size should be used to hold the lamp black 
together. About three drops of such size, as may be had 
by dipping the point of a lead pencil about half an inch 
into the gold size, will be found right for the above quan¬ 
tity of lampblack; it should be added a drop at a time, 
however. After the lampblack and size are thoroughly 
mixed and worked, add twenty-four drops of turpentine, 
and again mix and work. It is then ready for use. Ap¬ 
ply it thin with a cameks hair brush; and when it is 
thoroughly dry, the articles will have as fine a dead black 
as they did when they came from the optician’s hands. 


16 


236 


HAND-BOOK FOR OPTICIANS. 


VI. Blue Eyes.—It is said that all the Presidents of 
the United States, except Gen. Harrison, had blue eyes. 
Among the great men of the world blue eyes appear to 
have been predominant. Socrates, Shakespeare, Locke, 
Bacon, Milton, Goethe, Franklin, Napoleon and Hum¬ 
boldt, all had blue eyes; also Bismarck, Gladstone, 
Huxley, Virchow, Buchner, and Kenan. 

VII. Effect of Colors on the Mind. — An Italian 
physician lately made some interesting experiments in 
the treatment of lunatics. One patient was so melan¬ 
cholic that he refused all food. The physician trans¬ 
ferred him to a red-colored, well illuminated room, and 
after a stay of three hours the melancholy had changed 
to an unrestrained gaiety, and the patient made no fur¬ 
ther objection to eat and drink. Another lunatic who was 
wildly excited, was brought into a blue room where he 
soon calmed into a tractable being. All other means 
tried before had failed. 

VIII. Blindness Due to Decayed Teeth. — Dr. Wid- 
mark, a Swedish surgeon, having as a patient a young 
girl in whom he was unable to detect the slightest path¬ 
ological changes in the right eye, but who was yet com¬ 
pletely blind on that side, observing considerable defects 
in the teeth, sent her to a dental surgeon, who found 
that all the upper and lower molars were completely de¬ 
cayed, and that in many of them the roots were inflamed. 
He extracted the remains of the molars on the right side, 
and in four days’ time sight began to return, and on the 
eleventh day after the extraction of the teeth it had be¬ 
come quite normal. 

IX. To Increase the Strength of a cx lens, it is ne¬ 
cessary to remove it from the eye. With a concave lens 
it is the reverse; its removal from the eye makes it 
weaker. A cc lens is strongest the nearer we approach 
it to the eye. If, therefore, a nearsighted person com¬ 
plains that his glasses fatigue the eyes, and trouble 
his vision, but that he sees more distinct when they are 
a little removed from the eyes; then we have to decrease 
the strength of his glasses one or more numbers, even 
at the expense of sharp vision for the first few days, till 


MISCELLANIES. 


237 


the eyes are free from the overstrain of the former 
glasses. 

X. Direct Vision is that which pertains to the very 
center of the eye; that which belongs to the rest of the 
eye is called indirect or peripheral vision . Indirect vis¬ 
ion, although it may be very indistinct and imperfect in 
comparison with central vision , is, however, not less im¬ 
portant than the latter. Without peripheral vision we 
would be in the condition of a man looking through a 
long, narrow tube which would allow of his seeing noth¬ 
ing but the object to which the axis of vision was directed. 
It would be impossible for him to see objects to one side 
without an incessant turning of the head. 

XI. How Science Advances. —He who wishes to 
keep abreast with the march of science to-day must go to 
the work shop and into the dark corners of private labor¬ 
atories; for investigators rarely have time to write, so 
that text books are years behind the science itself. 



CHAPTER XXIX. 


Glossary. 


Aberration (Latin). The deviation of light from the 
straight line. 

Albino (Italian, whitish). One who lacks pigment in 
the skin, hair and eyes, therefore displays peculiar white¬ 
ness of the skin and hair, and a redness of the iris and 
pupil of the eye. 

Alkali (Arabic, the soda-plant). A name given to 
certain substances, such as soda, potash and the like, 
which have the power of combining with acids to form 
salts. 

Amuitrosis (Greek, obscuration). Impairment or loss 
of vision from some undetectable cause,—see Amblyopia. 

Ametropia (Greek, ametros , out of measure, ops, the 
eye). Is that condition of the eye, when parallel rays 
are focused either behind or in front of the retina. 

Amblyopia (Greek, amblys , dull, ops, eye). Impaired 
vision from defective sensibility of the retina. 

Anatomy (Greek, ana temuein, cutting up, dissec¬ 
tion). The study of the different parts and the struc¬ 
ture of the body. 

Aperture . The opening of an angle. In good micros¬ 
copes and telescopes, their aperture is often exceeding 
an arc of one hundred and fifty degrees.- 

Aqueous Humor (Latin, aqua, water). A few drops 
of watery, colorless fluid, occupying the space between 
the cornea, iris and crystalline lens. 

Aphakia (Greek, a, not , phakos, lens). Absence of 
the crystalline lens; for instance, after the operation for 
cataract. 

Artery (Greek, aer, air and tereo, to keep). A vessel 
conveying the blood from the heart outward to the or- 




GLOSSARY. 


239 


gans; so called because the ancients thought these vessels 
contained air, as they are empty after death. 

Asymmetry (Greek, a , not, syn , with, metron , meas¬ 
ure, not in measure). This word is the opposite of sym¬ 
metry, which means that the several parts of a body, or 
thing, are in due proportion to each other; while asym¬ 
metry means that they are out of proportion. 

Asthenopia (Greek, a, not, sthenos , strength, ops , 
eye). The eye has no strength in its muscles; sometimes 
‘ ‘ weaksightedness. ’ ’ 

Atrophy (Greek, trephein , to nourish). A wasting 
away from defect of nourishment. 

Atropine (Greek, atropos , black,—the name of one of 
the Fates). A very poisonous vegetable alkaloid, ex¬ 
tracted from the plant Atropa Belladonna, the deadly 
nightshade; the extract crystallizes in long, white nee¬ 
dles. 

Axis [Greek, axon , a straight line, real or imaginary, 
on which a body revolves, or may revolve]. In optics, 
a ray of light from any object, which falls- perpendicu¬ 
larly on the eye, called the optic or visual axis. 

Bi-focal. A lens having two different foci. 

Binocular [Latin, bini, two and two, oculus , eye]. It 
signifies an instrument used by both eyes at once. 

Binocle (French). Eyeglasses for both eyes. 

Brain . The mass of nervous substance contained in 
the cavity of the skull. 

Calorie (Latin, calor , heat). The principle of heat, 
the agent of heat and combustion. 

Canthus (Greek, canthos , the rim of a wheel). Angle 
of the eye; the inner and outer corners, where the eye¬ 
lids join. 

Capillaries (Latin, capillus, hair). The smallest 
blood vessels between the arteries and the veins, so called 
from their minute or hair-like size. 

Cartilage (Latin, cartilago). A firm, elastic sub¬ 
stance, like India-rubber, forming a part of the joints, 
wind-pipe, nostrils and ears. 


240 


HAND-BOOK FOR OPTICIANS. 


Cataract ( Greek, catarasso , to throw down, to break 
or disturb). Opacity of the lens or its capsule. 

Catoptric (Greek, catoptron , mirror). That part of 
optics which explains the properties of reflected light, and 
particularly that which is reflected from mirrors or pol¬ 
ished surfaces. 

Cavity (Latin, cavus , hollow). A hollow, inclosed 
space. 

Cerebellum (Latin, diminutive of cerebrum , brain). 
The little brain situated at the back and lower part of 
the head. 

Cerebrum. The brain proper, occupying the entire 
upper and front part of the skull. It is nearly divided 
into two equal parts, called hemispheres, by a cleft ex¬ 
tending backward from the front part of the head. 

Choroid (Greek, chorion , skin, eidos , form). A 
brownish-black membrane forming the middle coat of the 
eyeball. 

Choroiditis. Inflammation of the choroid. 

Cilia (Latin). Eyelashes. 

Concave (Latin, concavus , hollow). Curved or rounded, 
like the inside surface of a hollow globe. 

Congestion (Latin, con, together, gero , to bring). An 
unnatural gathering of blood in any part of the body. 

Contraction (Latin, traho , to draw). The active short¬ 
ening of a muscle or muscular fibre. 

Convex [ Latin, conveho , to bring together]. Curved 
or rounded, like the outside of a globe. 

Cornea [Latin, cornu , horn]. The transparent, horn¬ 
like substance which covers the front part of the eyeball, 
through which the light passes. 

Crystalline Lens [Latin, crystallum, ice,]. A trans¬ 
parent, circular body, rounded on its front and back sur¬ 
faces, situated in the eyeball, just behind the pupil and 
iris. 

Deviation (Latin, de, from, via, way). A turning 
aside from the right way or line. 


GLOSSARY. 


241 


Dialyte (Greek, dia and lyo , to loosen, to separate). 
A telescope in which the flint and crown glass of the ob¬ 
jective lens are not glued together, but mounted sepa¬ 
rately, leaving some space between them. 

Diaphragm (Greek, diaphragma , partition). A plate 
with a circular opening, used, in instruments, to cut off 
marginal portions of a beam of light. 

Diffraction (Latin, diffringo, to break in pieces). A 
change which light undergoes, when, by passing near the 
border of an opaque body, it forms parallel bands or col¬ 
ored fringes. 

Dioptric (Greek, dioptomai, I see through). That 
branch of optics which treats of the refraction of light 
and the properties of lenses. 

Diplopia (Greek, diplos , double, ops , eye). Double 
vision. 

Dispersion (Latin, dispar go, to scatter). The sepa¬ 
ration of light into its different colored rays. 

Dissolving views , are produced by two magic lanterns 
of equal strength, whose foci are centered on the same 
spot of the canvas on which the picture is shown. By a 
skillful manipulation of the adjusting screws, one picture 
ma}^ gradually disappear while another almost instantly 
takes its place. 

Distance (Latin, disto , to stand apart). Kays coming 
from a point nearer than twenty feet are divergent, and 
are considered as coming from a “finite distancebut 
rays coming from a greater distance than twenty feet are 
practically parallel, and are considered as coming from 
an “infinite distance.” 

Duct (Latin, duco , to lead). A narrow tube, usually 
designed to convey away a secretion from the gland in 
which it is produced. 

Elasticity (Greek, elastreo , to impel, or elao , to drive). 
The property of bodies by which they recover their for¬ 
mer figure or size, after the removal of outside pressure 
or force. 

Emmetropia (Greek, metron , measure, emmetros , in 
measure, ops , eye). The condition of the eye, when par- 


,0 


242 


HAND-BOOK FOR OPTICIANS. 


allel rays are brought to a focus upon the retina without 
any effort of the accommodation. 

Focus (Latin, hearth, fire-place). A point in which 
the rays of light meet, after being reflected or refracted. 

Glaucoma (Greek, glaucos , sea-green). A most se¬ 
rious disease of the eye, not well understood, but charac¬ 
terized by hardness of the globe, dilatation of the pupil, 
and often by a greenish opaque appearance of the pupil. 

Goggles (this word is of Welsh origin, gogelu , to shun, 
to shelter; the French, coquille is only a poor substitute 
for the same word). Protection spectacles of colored 
glass in the shape of a muschel or a hollow watchglass. 

Granulation (Latin, granum , grain). The process of 
forming small grain-like swellings on the tender mucous 
membrane of the eyelid, a disease; also the natural pro¬ 
cess by which the surfaces of ulcers and sores are cov¬ 
ered with new tissue,-granulation tissue or granulations. 

Horopter An obsolete denomination for Range of Vis¬ 
ion. 

Humor (Latin). Moisture; the humors are transpar¬ 
ent contents of the eyeball. 

Hyperaemia (Greek, hyper , over or above, haima, 
blood). An active superabundance of blood in an organ, 
or part of the body. 

Illusion (Latin). A deception of the sense (sight) or 
brain. 

Indentation (Latin, in, and dens, a tooth). A notch 
in the margin of anything. 

Inflammation (Latin, flctmmo, to flame). A peculiar 
diseased condition of any part of an animal body, char¬ 
acterized by redness, swelling, heat, pain and' febrile 
symptoms; there is first hyperaemia and then congestion. 

Ingredient [Latin, ingredi, to go into]. That which 
enters into a compound as one of its constituents. 

Iris [Latin, the rainbow]. The thin muscular ring or 
curtain which lies between the cornea and crystalline lens, 
and which gives the eye its brown, blue or other color. 


GLOSSARY. 


243 


Iritis. Inflammation of the iris. 

Irradiation. The phenomenon by which a brilliant 
body (especially on a dark ground) appears larger than 
it is, by reason of the stimulation of the light force, ex¬ 
tending over a larger area of the retina than that occu¬ 
pied by the image of the body. 

Kaleidoscope (Greek, halos , beautiful, eidos, form, 
sJcopeo , to see). An instrument which, by an arrange¬ 
ment of reflecting surfaces, exhibits an infinite variety 
of beautiful colors and symmetrical forms of its contents. 

Latent (Latin, lateo , to lie hid). Concealed, secret, 
hidden; not visible or apparent. 

Lens (Latin). A piece of transparent glass, or other 
substance, so shaped as either to bring together or dis¬ 
perse the rays of light. 

Ligament (Latin, ligo, to bind). A fibrous band or 
cord, serving to attach two bones to one another. 

Manifest [Latin]. Clear, disclosed, apparent, evident. 

Membrane [Latin, membrum , a limb or member]. A 
thin layer of tissue serving to cover some part of the body. 

Meniscus [Greek, meniskos , a little moon]. A lens 
convex on one side and concave on the other. 

Mica [Latin, mico , to shine]. A transparent mineral 
capable of being cleaved into elastic plates of extreme 
thinness. It is a poor conductor of heat. 

Microscope [Greek, milcros , small, skopeo , to look at]. 
An optical instrument which magnifies objects. 

Mirage [Latin, miror , to admire]. An optical illusion 
arising from an unequal refraction in the atmosphere, 
and causing remote objects to be seen double, as if 
reflected in a mirror, or to appear as if suspended in the 
air, like the “Fata Morgana.” 

Monocle [Latin, monoculus , one-eyed]. A single eye¬ 
glass. 

Motor [Latin, moveo, motum , to move]. Causing 
motion; the name of those nerves which conduct to the 
muscles the stimulus which causes them to contract. 


244 


HAND-BOOK FOR OPTICIANS. 


Muscoe volitantes [Latin, musca , a fly, volito , to fly 
about]. The appearance of grayish motes apparently 
before the eyes. 

Mucous Membrane . The thin layer of tissue which 
covers those internal cavities or passages which com¬ 
municate with the external air. 

Mucus [Latin], The sticky fluid which is secreted 
by mucous membranes, and which serves to keep them 
in a moist condition. 

Muscles [Latin, mus , mouse, musculus , a little mouse]. 
A band of fibres acting as an organ of motion in animal 
bodies. The voluntary muscles act in obedience to the 
will, and contract suddenly; the involuntary muscles do 
not obey the will, and contract or relax slowly. 

Mydriasis [Greek]. The unnatural dilatation of the 
pupil. 

Myopia [Greek, myo, to shut, ops , the eye]. Near¬ 
sightedness. 

Myosis [Greek], the unnatural contraction of the pupil. 

Nerve (Greek, neuron , a cord or string). A glisten¬ 
ing, white cord, connecting the brain or spinal cord with 
some other organ of the body. The nerves are the tele¬ 
graph-wires of the body. 

Neuralgia (Greek, neuron , nerve, algos , pain). A 
peculiar pain of a nerve of common sensation, not pre¬ 
ceded or occasioned by any other disease. 

Objective lens. The lens of an optical instrument 
which is directed to the object to be seen. 

Observatory. A place or building for making observ¬ 
ations on the heavenly bodies. 

Ocular lens. The lens of an optical instrument through 
which the eye looks. 

Oculus dexter (Latin, abbreviated O. D.). Right eye. 

Oculus sinister (Latin, abbreviated O. S.). Left eye. 

Ophthalmia (Greek, ophthalmos , the eye). Inflam¬ 
mation of the eye. 


GLOSSARY. 


245 


Ophthalmology (Greek, logos , a discourse). The sci¬ 
ence of medicine and surgery concerning the eye. 

Ophthalmoscope (Greek, skopeo , to examine). The 
instrument for exploring the interior of the eye. 

Optic (Greek, opto , to see). Pertaining to the sense 
of sight. 

Opticus (Latin) Optician. 

Optometer (Greek, ops , eye, metron , measure). Eye- 
measure; an instrument for measuring the limits of 
direct vision. 

Organ (Greek, organon , an instrument). Any part 
of the body which is adapted to perform a particular 
service, such as the eye, etc. 

Oxide. A compound of oxygen and a base. 

Oxygen (Greek, oxys, sharp, gennaein , to bring forth). 
A gas forming one fifth part of our atmosphere, and 
essential to respiration. 

Panorama [Greek, pan , all, orama , view]. A picture 
presenting from a central point a view of objects in 
every direction. It is lighted from above, and viewed 
from a platform in the center. 

Pantoscopic , is the Greek name for double-focus, or 
so-called Franklin glasses. 

Papilla [Latin]. Minute projecting filaments, being 
the termination of nerves, as on the tongue, also on the 
retina. 

Parabola. A conic section arising from cutting a cone 
by a plane, parallel to one of its sides. 

Paralysis [ Greek, paralyo , to loosen, dissolve or 
weaken]. An abolition of the functions of motion. 

Perimeter [Greek, peri, about, metron , measure]. An 
instrument to measure the field of vision. 

Periphery [Greek, peri, around, phero, to bear]. The 
circumference of a circle. 

Periscopic | Greek, ^m*, around, skopeo , to look]. To 
look about, a term applied to concavo-convex lenses. 



246 


HAND-BOOK FOR OPTICIANS. 


Phantasmagoria [Greek]. A magic lantern, or its 
representations. 

Phenomenon [Greek]. Anything visible, being pres¬ 
ented to the eye by observation or experiment; an ap¬ 
pearance whose cause is not immediately obvious. 

Photophobia [Greek, phos, light, pliobeo, to dread]. 
Intolerance of light. 

Pigment [Latin, pingo, to paint]. Coloring-matter. 

Pince-nez [French, pincer , to press, nez , nose]. Pin¬ 
cers, eyeglasses. 

Polarization . A change produced upon light by the 
action of certain media, by which it exhibits the appear¬ 
ance of having polarity or poles, possessing different 
properties. 

Polyopsia (Greek, polys , much). Seeing more objects 
than are present. 

Presbyopia (Greek , presbys, old). Old sight. 

Punctum proximum [Latin]. The nearest point of dis¬ 
tinct vision. 

Punctum remotum. The distant point of distinct vision. 

Pupil [Latin, pupilla\ . The central, round opening 
in the iris, through which light passes into the eye. 

Pange of Vision. The horizontal distance at which 
the eye is still able to discern objects. 

Reflector [mirror]. A telescope in which the rays of 
an object are received by a mirror, and from it reflected 
to the*magnifying ocular lens. 

Reflex action. An involuntary action of the nervous 
system, by which an external impression conducted by a 
sensory nerve is reflected or changed into a motor impulse. 

Refractor. A telescope in which the rays of an object 
are received and magnified by a set, or row of refracting 
lenses. 

Retina (Latin, rete , a net). The membranous expan¬ 
sion of the optic nerve in the interior of the eyeball, 
which receives the impressions resulting in the sense of 
vision. 

Retinitis. Inflammation of the retina. 


GLOSSARY. 


247 


Sclerotica [Greek, shleros , hard]. The tough, fibrous 
outer coat of the eyeball; the visible portion is the 
“white of the eye.” 

Sensation ( Latin, sensits, sense). The conscious per¬ 
ception of an external impression by the nervous system; 
a function of the brain. 

Spasm (Greek, spasmos , convulsion). A sudden ^vio¬ 
lent and involuntary contraction of one or more muscles, 
or muscular fibres. 

Specfroscope. An instrument to decompose light by 
means of prisms, which is used in the researches of 
Spectrum Analysis. 

Speculum (Latin). A mirror, either plane, convex 
or concave. 

Staphyloma (Greek, staphyle , a grape). A projection 
of some part of the eyeball,' either of the cornea and 
iris (Staph, anterior), or of the sclerotica and choroid 
(Staph, posterior). 

Stenopceic Slit. A blackened metal plate with a narrow 
slit in the middle, to detect the faulty meridian of an 
astigmatic eye. 

Stereoscope (Greek, stereos , solid, skopeo , to see). An 
optical instrument for giving to pictures the appearance 
of solid forms as in nature. 

Strabismus (Greek, strabos , twisted). The squinting 
of an eye. 

Striated (Latin, strio, to furnish with channels). 
Marked with fine parallel lines. 

Symptom (Greek, syn, with, pipto , to fall). A sign 
or token of disease. 

Tapetum (Latin, tapis , tapestry). A shining spot to 
the outer side of the optic nerve in the eyes of certain 
animals, due to the absence of pigment. 

Temple (Latin, tempus , time, tempora , the temples). 
The part of the head between the ears and the forehead; 
so-called because the hair begins to turn white with age 
in that portion of the scalp. 


248 


HAND-BOOK FOE OPTICIANS. 


Tissue. Any substance or texture in the body formed 
of various elements; such as cells, fibres, bloodvessels, 
etc., interwoven with each other. 

Transparent (Latin, trans, through, pareo , to appear). 
Capable of allowing light to pass through. Transparent 
bodies can be seen throngh. 

Vein (Latin, vena). A vessel serving to convey the 
blood from the various organsJ^oward the heart. 

Vibration (Latin, vibro , to move to and fro). Quick 
motion to and fro. 

Vision (Latin). The faculty or act of seeing external 
objects; the symbol is Y. 

Vitality (Latin, vita, life). The state or quality of 
being full of life. 

Vitreous (Latin, vitrum, glass). Having the nature or 
appearance of glass. 


INDEX 


Aberration, chromatic, 102. 

“ spherical, 105. 
Accommodation, 113, 119, 145. 
Achromatic lenses, 104, 107. 
Achromatism, 102, 114, 191. 
Acuteness of vision, 90, 172. 
Aluminium, 20, 233. 

Ametropia, 117, 238. 

Anatomy of the eye, 108. 

Angle of incidence, 99, 141. 

“ reflection, 141. 

“ refraction, 99. 

“ vision, 91. 

Aqueous humor, 109, 238. 

Artificial human eye, 156. 
Astigmatism, 134. 

Axis in cylinders, 48, 69. 

“ in pebbles, 35. 

“ of vision, ISO, 183, 239. 

Base of prism, 44, 56. 

Binocular ophthalmoscope, 144, 223. 
Blind spot, 112. 

Brachymetropia, 127. 

Caloric rays, 162, 170. 

Camera obscura, 108. 

Candle, 169. 

Canthus, 159, 239. 

Caruncle, 159. 

Cataract, 148, 240. 

Choroid, 110, 112, 240. 

Chromatic aberration, 102, 105. 
Ciliary muscle, 113, 124. 

Coddington lens, 193, 204. 

Colors, 83, 100. 

“ harmony of, 85. 
Combinations, converting, 50. 
Commissure, 110. 

Complementary colors, 85. 
Compound lenses, 53, 65. 

“ “ measuring of, 66. 

“ “ use of, 140. 

Conjunctiva, 111. 

Conversion of cross-cylinders, 50. 
Coquille, 87, 242. 

Cornea, 109, 111, 240. 

Corundum, 233. 


Cross-cylinders, 49, 53. 

Crown glass, 24, 33, 36. 

Crystalline lens, 112, 136, 145, 240. 

“ “ capsule of, 113. 

Crystals, single refracting, 34. 

“ double “ 34. 

Cylindrical lenses, 47, 136. 

Decentered Lenses, 56. 

Dialytes, 107, 218, 241. 

Diamond Oil, 63. 

Diaphragm 105, 109, 241. 

Diffraction, 213, 241. 

Diopter, 11. 

Diploma, 124. 

Dispersion of light, 99, 103, 241. 
Double focus glasses, 76. 

Double refraction, 32, 34. 

Electric light, 166. 

Emission theory, 96. 

Emmetropia, 117, 242. 

Equivalents, 50, 140. 

Ether, 96, 166. 

Expressive eye, 182. 

Eyeball, 108. 

Eyebrows, 179. 

Eye-killers, 14. 

Eyelashes, 180. 

Eyelids, 180. 

Eye sharpener, 136. 

Facial expressions, 179. 

Fakes, 27. 

Flint glass, 23, 106. 

Fluid for drilling, 63. 

Focus, negative, positive, 16, 47, 242. 
Franklin glasses, 77, 210. 

Gas light, 167. 

Glass, pliable, 184. 

Glass, drilling, 63. 

Goggles, 77, 242. 

Harmony of colors, 85. 

Height of lighthouses, 172. 
Hypermetropia, 117, 121. 

“ absolute, 124. 

“ latent, 123. 




250 


HAND-BOOK FOR OPTICIANS 


Inch system, 12. 

Incidence, angle of, 99. 

Index of dispersion, 86, 100. 
Index of refraction, 36, 40, 99. 
Injuries of eye, 152. 
Interference of light, 213. 
Invention of spectacles, 184. 
Inventions, 27, 198. 

Iris, 109, 242. 

Leech, artificial, 216. 

Lens, Arundel, 36. 

“ cylindrical, 47, 135. 

“ decentered. 56. 

“ human, 112, 114. 

“ interchangeable, 58. 

“ spherical, 46, 243. 

Lens measure, 43. 

Light, 96, 162. 

Luminosity of eye, 141. 

Macula lutea, 112, 116. 
Measurement of prisms, 43. 
Meniscus, 16, 243. 

Meridian, faulty, 138. 

Metric system, 12, 17. 
Microscope, 107, 192, 225, 243. 
Muscse volitantes, 112, 244. 
Muscles of eye, 57, 111. 
Mydriatic, 124, 140, 146. 
Myopia, 118, 126, 244. 

Myopia in distans, 132. 

Nachet, trial frame of, 49. 
hi ear-point, 119, 246. 

Normal eye, 116, 119, 122. 
Nose-guard, 73. 

Nose-pieces, 72. 

Oil-lamps, 167. 

Old sight, 119, 120. 

Opera glass, 195. 
Ophthalmoscope, 141, 245. 
Optical center and line, 55, 56. 
Optical scale, 100. 

Optic nerve, 111. 

Opus majus, 186. 

Orbit of eye, 108. 

Pantoscopic spectacles, 78, 245. 
Pebbles, axis, non-axis, 35. 
Perfected spectacle, 54. 
Perfection bi-focals, 81. 
Periscopic lenses, 16, 46, 245. 


Petroleum light, 168. 

Pigment, 112, 246. 

Plane lens 40. 

Polarization, 202, 219. 
Polarizer, 34. 

Presbyopia, 119. 

Prismometer, 44. 

Prism, 41. 

Progressive myopia, 129. 
Protection spectacles, 82. 
Protractor, 42. 

Pupil, black, 109, 111, 142. 
Pupil, distance, 71. 

Pyramidal muscle, 176. 

Quality of lenses, 19, 23. 
Quartz, 20. 

Radiating fibres, 113. 

Radiating heat, 165. 

Radius of curvature, 14. 

Range of vision, 90, 173, 246. 
Recti muscles, 57, 132. 
Redressing frames, 88. 
Reflection, 141, 185. 

Reflector, 104, 246. 

Refraction, angle of, 99. 

“ double, 34. 

“ index of, 36, 40, 99. 
Relief to injured eyes, 152. 
Retina, 110, 112, 116, 246. 

Rock crystals, 20, 31, 189. 

Rods and cones, 110. 

Sclerotica, 112, 247. 

Second sight, 147. 

Secretion of tears, 176. 

Senile changes in eyes, 149 
Shears, English, 60. 
Short-sightedness, 127. 
Silicium, 20. 

Snow blindness, 87. 

Spectrum, 36, 82, 100, 103. 

Split glasses, 78. 

Squint, 124, 132. 

Standard sizes of lenses, 58. 
Stanhope lens, 194. 

Staring look, 160, 181. 
Stenopaic slit, 87, 127, 247. 
Sties, 125, 136. 

Strabismus, 124, 247. 

Tears, 175. 

Telescope, oldest, 188. 
Temperature of Universe, 165. 





INDEX 


251 


Test-types, 90, 133. 

Tinted glasses, 82. 

Trial box or ease, 66. 

Trial frame, 49. 

Undulatory theory of light, 96. 

Velocity of light, 97. 

Vibration, 97, 166. 


Vision, direct, 116, 237, 248. 
Vitreous humor, 112, 248. 

Waterglass, 21. 

Waves, aerial, 100. 

“ ethereal, 100, 166. 

“ horizontal, vertical, 138 

Yellow spot, 112. 


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